Not aging gracefully

I’m almost 63. I’ve been very fit my entire life. I’ve also always been into working out. Beginning about a year ago I started putting on weight. It has increased dramatically. However, my eating habits have not changed. In fact I’ve become even cleaner as I’m fighting this weight increase. But now I can eat clean for a week and lose 1/2lb. Have a slice of pizza one time and I gain 3.
I’ve had my blood tested and everything is fine. This is very discouraging. I’m not sure how to reverse this change.