Metabolically, everyone is different, which is a good thing. Some people can pack on muscle without doing much, others struggle to gain anything. Lance had to lose 7 kilos of protein for his TdF comeback. Similarly with fat, some people overeat just a tiny bit and on it comes, other people can stuff themselves every meal and stay skinny. The only person who can really advise you is a personal coach or yourself, since you are apparently self-coached. You measure your waist but that's all? Why is that the only important measurement? At least also measure your chest, biceps, and thigh and chart them. You can't figure out what's going on without data.

Speaking of data, you probably have systematic errors in calorie intake and/or expenditure. Otherwise you couldn't gain weight. So take that as a given. Too bad you didn't keep track of calories during your period of weight loss, so you don't really know which it is or maybe it's both.

My guess is that you substituted protein for some of your carb cutback and now you're putting on muscle. Maybe when you were losing, you were losing both fat and muscle because your diet was too low in protein. Who can know without data? Besides calories in, you need to track your macros.

My system is not to track calories at all, only my weight. If I'm gaining, I eat less but keep the protein up where it needs to be, which is well over the minimum for youngsters. You can't ride it off. You can only eat it off. So ride as much as you want - more is better - but manage your weight with your diet. Today I weighed 6 lbs. more than when I was a somewhat skinny freshman and rode the hell out of my road bike. I rode RAMROD this past Thursday and weighed 10+ lbs. less than when I last rode it solo back in '04. No calories, just smaller portions.