I had exactly what you described from around 16 weeks with Tristan, around 18ish weeks I started losing my plug. The OB was not concerned at all as the mucous plug itself can re-generate.

The back pain worries me a bit, but if it was a once off then I probably wouldn't worry. If it does continue be sure to get checked out. Better to lose a few hours and know everything is ok than sit worrying.