I can understand your concern about mastitis. How full does the right side get? If you feel full and uncomfortable, it is best to express, but only enough for comfort. Otherwise your supply won't settle down as quickly. Over the next couple of weeks it is likely that your supply will start to settle down, and by 12 weeks you might even find that you don't feel full at all between feeds. This is totally normal, and still means you have enough milk. Are you comfortable with expressing?

With the thrashing, is it only on the right side that she does it? It could be that finds it harder to draw that nipple out, it could be waiting for let down or it could be the fast flow. It might even not be related to you at all, but she might just be more comfortable lying one way rather than the other. Have you tried different feeding positions - maybe the football hold, or feeding lying down would work better on that side.

It's really confusing at this early stage, but it does get easier.