There is no risk from the mutual masturbation.
It sounds like you were unclothed for the frottage? Unclothed, skin to skin contact with heavy grinding or rubbing can transmit syphilis, HPV and herpes.
However, a very small bump is unlikely to be any of those things, especially a week after. Syphilis causes a red, round, open sore that doesn't appear for at least 10 days. Herpes causes clusters of blisters, and HPV causes white or pink bumps that usually don't appear for a month, at least.
If you are concerned, get a doctor to check it out. It doesn't sound like an STD, but only a doctor can tell you for sure.