Is a BH contraction possible at 18 weeks?

I am 18 weeks pregnant. I was just lying in bed and I just felt kind of strange down in my pelvic region - below my belly on where my uterus is. It was really really hard. It only lasted less than a minute. Right now it is not hard. Is that a BH contraction. I didn't know I could notice them so early. I know they start as early as 6 weeks but most women don't notice them until much later. Possible? If not, why did my uterus do that?