Ay common yan sa pregnancy, lalo na pag may constipation or gas dahil sa iron supplements, a few of us moms na kumukuha ng ferrous noticed the same thing, parang hardening lang talaga minsan especially pag puno ng tyan. The diarrhea last week could've also triggered some cramping, but since you're 20 weeks na, best check with your OB just to make sure lahat normal lang, especially if may pain or bleeding, better safe than sorry!