If you started lifting in the last three months, you probably gained more mass in the legs than anywhere else. Second largest muscle group would be the back. So your body is adjusting to those proportions.
This may be true for most people depending on their athletic background. I grew up playing soccer and lacrosse, lots of running and sprinting. In junior high I started running cross country and track. I neglected building upper body strength and became very very powerful in the lower body. I then started training kung fu and tai chi, and became even more powerful in my legs due to daily horse stance! So now, my weight routine is mostly upper body to make up for this imbalanced training.
I do a ton of calisthenics, but have yet to build the upper body structure I'm going to be happy with. I have definitely gained upper body mass due to all the compound movements of the calistenthics, but now in order to really to fill out my upper body I think that heavy compound weight exercises are needed.
The calisthenics may be getting difficult for you because what you have been building is not FUNCTIONAL STRENGTH. This type of strength is the strength we see on gymnasts and pro athletes. Gymnastic strength conditioning for example promotes the ability to do the exercises you are having difficulty with. Wrestlers lift weights, but they also do a shit ton of dips, pull ups, pushups, and rope work. These guys are some of the strongest people out there.
Stop training solely for aesthetics and mass, and start training like a pro athlete is my advice. Form follows function, meaning you will reap an adonis physique by training for functional strength. Train just for mass and aesthetics, and you won't have functional strength.