Feel to see if it has any lumps on it. It sounds to me like it's just your test shutting down this happens to me sometimes toward the begging and end of a cycle.
check and see if you have a bump the size of a pee. if you do this could be testicular cancer and you ned to go and see a urologist immediately. if detected early this cancer is99% curable. if you have cancer they will have to remove your whole testicle. go have it checked immediately. if it is remove you will still be able to function normally.
dont fuck around with this one,,, you could be very sorry in the end,,, it also could be a hernia,,,, but I am no expert,, so get your shit checked out