You should get it checked out for sure. Testicular cancer usually becomes dangerous if you don't get it checked out because of embarrassment or ignorance. I had testicular cancer last year and had to have a round of radiation and a partial orchiectomy (i.e., they removed the nut). I casually brought it up to my wife who is a nurse and she started flipping out when she palpated my balls and felt a pea sized lump. Luckily I had no problems, caught it early.
It is extremely unlucky/uncommon to get it in both nuts at the same time though.