Pinching in cervix

Brittany • Chloe Elizabeth 7-30-13. Sophia Grace 10-15-15. Baby #3 09-20-16
I'm 19 weeks and for the last 5 minutes I've been feeling this pinching, sharp throbbing pain literally in my cervix that lasts for a few seconds, goes away, and comes back for a few seconds more. What is this??