No, you don't need a cup... lol.
Just work on your two point more and also make sure your stirrups are short enough. My jumper coach has us doing trot and canter laps around the ring without stirrups in two point. It is pure hell on the body but it will certainly make you stronger in the long run + help find your balance. Also make sure you're not throwing yourself out of the saddle over jumps. When some people are learning to jump they have a tendency to rise higher out of the saddle than necessary which also sucks for the horse since they end up slamming back down.
I suppose if you are getting hurt to the point where it's uncomfortable then a small cup might help until you're able to jump cleanly. I wouldn't advice wearing one in breeches though, it would probably look funny and bring unwanted attention to you from other riders. :P
Do you have the same problem when trotting jumps or just at the canter?
The hardest part isn't finding what we need to be, it's being content with who we are.