does it feel a bit harder than usual or feel like there is a pea under the nipple.
I had it when i was about 16 and turned out to be one of the early warning signs of testicular cancer (although they didnt know that at the time). Not saying youve got it, just aparently it was probably associated with an elevation in estrogen in my blood. so this could be a result of your cycle.
If I were you mate, have a nice warm bath tonight and just check your balls for any irregular lumps. lightly!!!!dont want you hurting yourself.
dont want to scare you but its better to be safe than sorry.