If your husband doesn't think your health is more important than free childcare, that's disappointing. I think even if these were your kids it wouldn't be unreasonable to find help. You are not their babysitter. You are a stepmom and anything you do extra is a favor. Not an expectation 

I was going to take a step back from babysitting the kids to use the lil energy I have for my self and my school work. 
aparently neither the kids mom or husband were happy with this.

Well, isn't that just too bad. Those kids are not your responsibility, even if your health was great.

Shame your selfish husband for not caring about your health.

Tell them to F- off and take care of their own children.   May shound harsh, but there is just too much of this--a divorced man and his ex- expecting other people, in particular dad's new wife or SO, to take care of their children.  And then when the new wife or SO says something like, "I can't handle this," they are made out to the meanie Evil uncaring SM.  Mom and dad will have to do what EVERY OTHER mom and dad has to do for THEIR children--figure it out vs. trying to bully and badger SM into doing it.  And even if SM does do it. . .  years down the road all she'll have is SKs who resent her for being there when their mom or dad should have been, a BM who will be resent her for being there period, and a DH who will resent her for not loving his kids more, as in 'putting up with their shiatsu the same way he does.'

Your husband told you you were his help? That's so wrong. Ok, sure, partners help each other out. But now he EXPECTS you to continue to do something based on a generous offer, even though circumstances have changed. That is selfish and rude, and I would not hesitate to call him out on it.

It sounds like you can't even properly care for these children based on the symptoms you described. And yet the parents are ok with this. And your DH isn't exactly supporting your recovery. Where is YOUR help? They both sound like craptastic parents.

And yet again, no good deed goes unpunished here in Stepville.

These kids have one BM and one BF.  It's up to them, who made the choice to have children.  To take care of those children.  Not you.  You do not become the babysitter because Bf and BM can't figure it out.  No reason to be mad at you. 
 

Time to disengage from all of this,  let them work it out. Hire a babysitter for the kids.  If DH is not home to take care of his kids. Those kids are at BM 

Perhaps your husband should consider a more local job if he can't handle the stress of not knowing where his kids are.

What is their custody arrangement?

In any event, it's not your job to watch their kids if you don't want to.  Since you had previously agreed to this arrangement, I think it's fair to give them some notice so they can make other arrangements, but agreeing once doesn't mean you've agreed to watch his kids for all eternity.  I think your husband is paying too much attention to his own convenience and not enough to your health.

Kids mom says he is supposed to help her with the care of the children and my husband says I am his help

First of all Dad isn't helping/parenting at all since he is a truck driver, you literally are "The Help." If dad isn't there you to do his job it should not fall on you.  He and BM made the kids so it's their job to find them care.  Wasn't there a recent discussion on ST about a "bang nanny?"  Sounds like this is the role you've landed OP.  

what am I supposed to do here. 

You have to prioritze yourself, obviously none of them care enough to help take care of you or even show concern for your well-being.  Like you I used to be a people pleaser to my own detriment. I would run myself into the ground to make others happy. I fave finally learned to take on what I can handle and say NO, I can't, I'm busy, Not available...etc to requests that I can't or don't want to do. In your case you have real health issues.  You can always make your doctor the "bad guy" here and say, "My doctor says I need to focus on my health right now and that requires rest, vitamins, a reduced workload and less stress."  Do whatever you need to to recover. Yes they're all going to be fussy because their expectations of you aren't being met but they will get over it.  It's unreasonable to expect you to fill in for dad for weeks at a time.  Youh health and well-being are important too!  

No..... you are his equity life partner!! Not a maid but a partner!!

you finishing your studies benefits you both as a couple, you being a free babysitter does not contribute to your quality of life

selfish bio mum is an ahole too!! If she wants her husband to help out with the kids, well he is!! Its caled child support and he needs to work to provide for it!!! 

Not only is bio mum selfish for wanting cs, she also wants free babysitting so she isn’t out of pocket during her time

also i have always had anaemia and going on iron pills doesn’t work right away. Even with an iron rich diet, it can take several months. Your husband obviously doesn’t care about you as a partner