Felt and heard a POP from the inside last night...

Jade

I was lying in bed trying to sleep, baby was wriggling loads like she usually does, but hen there was a big POP inside down below that I heard and felt. I shot up out of bed fearing my waters had broken but there was no fluid or anything. Any ideas what that could be? Was it just baby whacking a bone or engaging? I'm 35 weeks 4 days. Any advice appreciated xx