This is not going to help at ALL but I am just so impressed that you walk 8km at 30 weeks pg! OMG hun you are amazing!

Anyway, sounds like a cramp or stitch. Are you eating enough bananas? I get terrible leg cramps when my potassium is low, so maybe have a banana before your walk. Not sure what else it could be besides round-ligament pain, or just general pullying and stretching from bub.

Good luck babe, I hope it doesn't impede your exercise too much,