Okay so September to December 20lbs of MUSCLE, not total weight. So what is your total weight increase? Like 30-40lbs? We don't only gain muscle, by definition.

So let's say 20lbs in about 4 months, 5lbs of pure MUSCLE each month. Just over a pound a week, that's nice! I assume if you have 20lbs of only MUSCLE to build then you may be a beginner.