Michelle
I think you get them all the time it's just you don't notice early on.

I had one happening during my 13 week ultrasound so the bottom right hand bit of my uterus was all crunched in and bub looked like s/he was resting over one of those giant gym balls on his/her tummy. Not good for measuring nucchal fold! I read somewhere they are good things to have.

I've also been having the odd tightening sensation about once a fortnight since about 17 weeks.