I think it is from the testes going up and down in size. For me when they shrink while on I get aches. When I bring them back to size with HCG - human chorionic gonadotropin - and even on clomid they get sore too. Especially if I am sitting for a while. Not to be gross try to skeet if you can it relieves it for me. It can be an std but if you just did a cycle or are starting one 9 out of 10 times it is from the testes shifting. It is very common.