I had that last week, went on for a couple of days and then stopped. my MW sent me in for a scan just to have a look at the position of placenta and to look for any causes of bleeding and all was fine, just my cervix playing up. She said its pretty common, but they often do offer a scan, just for reassurance and also to screen for the tiny percentage who do have a placental problem.
I'd definitely ring and check, hope all goes well
