It could be tension, but as Scissor Queen said, most likely that you're moving too fast for the thread to keep up. I'm just learning too, and am happy that my new Janome has speed control on it. I'm finding that the faster speed is better for me, but I work on a practice sandwich before starting on the real quilt to make sure the tension and my movements are working properly. Keep practicing. It takes a while but it really does get easier and better looking over time.