I'm a 25 year old male and I added 20 pounds of fat in the last 3 months. My father had a similar experience at the same age, so I guess it's age-related.
Before my mid-20s, gaining muscle, actually any type of mass, was really hard for me, regardless of how much I ate or exercised. Now, I have the opposite situation, I can't lose weight. So, my idea is the following: instead of trying to lose weight, in my current situation, I should focus on gaining muscle mass.
So, how likely is it that I will gain muscle mass more easily now? Or is growing fatter not usually related to gaining muscle mass?