Sounds like a tension issue, but are you sure it's the bobbin thread that doesn't look normal? Usually if there's a problem on the back it's the top thread, and if the issue is on the top, it's the bobbin. I would rethread, using different colors in the top & bobbin sew a bit on a practice sandwich. this way you can pinpoint which thread is causing problems. If you can't resolve it, post a picture so we can get a better idea.