Co-worker

So I have this co-worker who is always asking for a day off. I'm pretty sure it's at least once a week now that she texts me and asks me to take a shift for her or trade with her. I'm 21 weeks pregnant, I dont have a car so I walk to work, then I'm on my feet the whole time I'm there and have to walk back home. IM TIRED! I'm tired all the time and I'm sick of telling her no. I dont say anything to my boss because I'm still relatively new (everyone else has been there for years compared to my one year). I'm honestly just trying to make it to January when I'll go out on maternity leave and then I'll decide if I'm even going back to that job or not. I dont want to seem like I'm using my pregnancy as an excuse but I'm lucky to be this far along in the first place (previous miscarriages and an ectopic) and I dont want to keep putting all this stress on my body. Anyway, I just wanted to get this off my chest