Testicle ache?

I am 15 years old male and for a few month a have felt an ache in my left testicle, it isn't painful as such and only occurs after i have exercised for a while as i am an active kid. I have felt around and the ache seems to be coming from the back of the testicle where the epidiymis is? there is a lump but also a lump on the right testicle in the same place yet that one does not ache. Do i have testicle cancer or a cyst or something? I have also recently been experiencing lower back pains is this related? As i said im only 15 and really scared.