Weight gain explanation? Just looking for clarification..

I'm lucky that my weight is in the healthy range so I should not obsess. But alas. I do at times. I have been working out 3-5 days a week for a long time. I use Kelli and Daniel's programs often so I'm well balanced in my routines. I eat clean pretty often. So here's the thing- I'm gaining weight. Ever so slowly. The number on the scale is going up. It's not muscle gain. No way. I've Ben consistent with my workouts for 4 years. I challenge myself with my weight lifting but I am not working like crazy to build Mass. I get sore and it feels good and I move on. So it's not muscle mass. I guess maybe I'm over eating? But I haven't changed my routines at all really. I mean the holidays didn't help. But my weight is up since after the holidays. After all the cookies and stuff. Anyway. Is it because I'm in my 40's now? Maybe my body is just changing how it reacts to food and exercise? I don't get it. It's just a bummer. Like I said. I'm not overweight at all. But I wish the scale wouldn't keep going up a pound this week and a pound the next. It's odd to me and a bit frustrating. Just venting and maybe looking for some insight. Thank you!