Early pregnancy and hard uterus?

I'm a couple days over four weeks and I've noticed my uterus feels like a hard ball. It isn't extremely noticeable unless I'm sitting and have to lean over. This may be tmi but my husband says he felt it the last time we had sex and that was Saturday night, he also said he felt it during my pregnancy with our son and the one I miscarried in May (was three months when I miscarried). I'm just wondering if it's normal since I'm just now noticing it. I didn't notice it the last two times.