Suggestions on Fiance/step kid dilemma?

I'm almost 6 months pregnant with my first baby. My fiance has 3 kids from previous relationships. He sees them every weekend.

I been sick for the past week, it started out as an itchy throat, and ended up being a full blow viral infection. I've gone to the Dr, and since it's a virus was told there isn't much they can do but let it run it's course. I'm miserable.

Two days ago, my fiance's ex GF called to say his youngest son was sick with the same thing that I have and that she was taking him to get antibiotics, only to call later to say that they wouldn't give him anything for it. Obviously. So my fiance and I both assumed that she would stay home with him this weekend as it's a 2 hour drive there just to pick him up not to mention it's cold outside. She called today however and asked him to pick him up early because she had plans. I feel bad, but I asked my fiance NOT to go get him and to ask that she please keep him this weekend since I feel like absolute poo. I can't breathe out of my nose, my throat is on fire, and i'm pretty sure someone planted a cottonfield in my lungs. My fiance got angry however, saying that it was his weekend with his son and he didn't think he should have to sacrifice the time with him. I don't either, but I work at a school and Spring Break ends this weekend so I go back to work on Monday. With a baby on the way, I can't afford to miss any more work and even though it makes me feel selfish I just don't want to spend my weekend with a 2 year old who on the best of occasions is whiny, mischevious, ornery and sometimes just downright mean.

My fiance's daughter from a different relationship will also be here but she's 8 and usually keeps herself occupied in her room on her computer or listening to music.

I mentioned going to my mom's for the weekend so that I can just relax and get better and that made my fiance even more angry.

I love my fiance, I love my stepkids, but I'm really worried about my health and the health of my unborn baby.

Do you think I'm wrong for wanting that or should I go ahead and migrate to my mom's for the weekend?

Also, my fiance wants to work overtime at his job tomorrow, which means he would get up at 4 and not get back until 10. My stepson wakes up every saturday at 6 and since he's two needs to be supervised, so that would mean I have to get up with him.