I don't know how far along you are, but if it changes size in the day it sounds like pregnancy bloat. It can be quite bump-like. Usually women don't get a baby bump until after 12 weeks (a little later with first babies and a little earlier with 2nd or more babies) as the uterus is still down below your pubic bone till that point. My bump didn't come until past 20 weeks, but I had some serious bloat before then that looked like a bump. Bloating can mimic a bump incredibly well from very early on, right up into second tri.
Drinking lots of water helps with pregnancy bloat.
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