I have a hardish mass about the size of a marble on the top of my left testicle it is sore when it is squeezed it's really smooth could this be a std? it feels heavy after it has been knocked

See a doctor: You need to provide a full history and be examined and perhaps tested by a professional. Cannot diagnose this over the internet.

Get checked NOW: This is probably not an std, but you need to be checked immediately anyway. You are in the age group at highest risk for testicular cancer. Hard lumps are worrisome. I don't mean to scare you; it probably isn't cancer, but you need to be checked to know for sure.

See your doctor: This problem can only be solved by seeing your doctor and being evaluated face-to-face. After a thorough examination, your doctor should be able to tell you what's wrong and what to do about it.