I've had my tummy harden for a minute or so and then go away and I spoke to my OB at my last appt and he said the uterus contracts and it's nothing to worry about.

but tonight I felt it harden in one spot and it felt quite hard and round, like a tennis ball was under my tum.

what was going on? any ideas? is this normal? it scared me. It went away after a couple of minutes. I'll be on the phone to the OB's rooms tomorrow to talk to one of the nurses but until then thought I'd come here .

I'm 21 weeks.