Is your swimsuit polyester or lycra? I usually get some of the rub marks from polyester swimsuits. If it's lycra, my bet would be that it needs to be a bit smaller. As the suit wears out and looses it's elasticity, it will move a lot more. Usually swimmers will tell you to wear 1-2 sizes smaller than you measure, but if it's a polyester suit, then the tightness can actually make sores worse since they don't stretch much at all. I stick to a polyester suit though because they last so so so much longer than the lycra ones.

Hope this helps