I'm 37 weeks as well and I have been getting that feeling for like over a month now. When I told my doctor I thought my daughter was knifing me she said its a common complaint in the 3rd trimester and just another sign that things are progressing the direction you want it to. It from like FreshLove said the baby putting pressure on your cervix and moving around down there. I know that my daughter just seems to love grinding her head on it when she is super active. Hurts and makes me honest to god think she is going to fall out or make my water break. Ugh. But despite how uncomfortable it is it doesn't mean you'll have baby in the next couple days or even weeks but it does mean that things are happening!
It is your baby changing positions and pushing/sitting on your pelvis. It's pretty common in the 3rd trimester as baby gets bigger.