Exhaustion + Work

I'm 22 weeks. I work in early childhood with ages 2-3. The hours are long + the work is hard on my back. I would love to cut down on my hours, but my boss does not allow this to happen without a specific medical condition. I have taken 3 days off of work since January because of pain back + contractions (Braxton hicks?) that come about when bending down to feed/help/change/play with the kids. I've spoken to my doctor and she believes it's temporary fatigue + should just take off the days when I deem necessary, but I'm afraid of losing my job. We NEVER have any vacation days, + I know my boss will try + replace me at any chance she gets. Anyone else struggling with this or something similar? Any suggestions?