Keep an eye for any discharge, like if you're spotting. If you start spotting keep track of how much you're bleeding. Go to the ER if it worsens.
I think its common at this stage. I'm 26 weeks & get that if I stand too long or over do it. This is my 4th pregnancy.
It MAY have been, but it could also be your muscles stretching. If it occured with sudden movement it was most likely your muscles screaming over the sudden tension.