Paul said that you may want to look at your diet, specifically how many calories you're eating. If you put on a lot of muscle mass, your basal metabolic rate is going to increase, he explained. This, in simple terms, is the amount of calories your body burns on its own at rest. He continued, "You'll be burning more calories throughout the day, and then you're going to have to eat more calories."
Everyone's bodies are different, so you can use this equation as a guide for calculating how many calories you should be eating per day. But, for muscle gain, you generally need to be eating in a caloric surplus.
As we've mentioned in the past (and countless times at that), when it comes to what you're putting into your body, carbs and protein are important because they help build and repair muscles. Find out more information on proper macros here. Paul said, though, that you shouldn't be doing anything drastic; make small changes by bumping up calories slowly to see what works for you.