How much "mass" did you gain?
PIn regards to the forearm pain you described, it sounds like forearm pump to me. Have you tried wrist curls? If so how do these feel? Forearm pump is a *****! as it completely kills your grip so would recommend it at the end of a workout!
It seems to be more in my left forearm than my right. The guy in the gym said it was something to do with the 2 parallel bones in your arm which force together when your arm tenses, thus applying outward pressure??