I got a treadle yesterday, and I was sewing along on it, and then the stitchs bubbled up on the bobbin side, and sort of of the top. The other half of what I sewed was just fine, so what can I do to adjust the tension? I was pumping it pretty hard, did it vibrate and change or something? Did I thread it wrong? It is I believe but am not sure, a singer 99, 99-13?