Stomach aches and stress

I'm a Sahm, every day it's my 4 year old son usually causes me no problems, but every Monday -Tuesday and every other Friday-Tuesday we also have my 2 step daughters, and being 7 months pregnant it just seems all 3 together just causes me so much stress lately.. It gets to the point my stomach will actually start to hurt from the stress cause they never ever want to listen to anything. And the 2 girls came over yesterday both with horrible colds, Lord only knows why their mom didn't just keep them instead of making them more miserable. I just feel like the further I get in my pregnancy the more stress I get from watching them all day all the time. None of old enough for school so I have them all from 630am till 4pm