Rapid Weight loss, Then Rapid Weight Gain. Kefir?

I'm 29, 5'6, and during most of my life I have been on average 130 pounds. I have had depression, and what I believe is an abnormal metabolism - I could eat anything I want and not gain a pound. Even with strict exercise and diet. I've been in sports all my life, and I've noticed that the physical load never got any easier.

Years ago, I was on a strict weight lifting program of 3 days a week. I'd consume massive amounts of food and protein past my appetite, which was the only way I was gaining. The highest I got during lifting, was 160. If I'd miss one meal, I'd be 2 pounds less the next day. After I stopped, my weight quickly dropped back to 135-140 range.

Right now I am 171.5. The only thing different I have been doing is drinking kefir whole milk. I've read lots about kefir, and I've found nothing that mentions weight gain, only weight loss. Is it possible that kefir has balanced out my metabolism?

The weight is very rapid. Last year I was 135, in November. I was shocked once I hit 160 (my all time high) then today I step on the scale and I'm over 170. I fear I'll be 180 in a few weeks. I've always wanted to put on weight, but this is a bit scary. My health has always been perfect. I've always been very muscular and strong for my size too. I seem to build muscle quite rapidly in my chest and shoulders especially. I don't eat a lot, and it's all healthy (whole grain, no processed sugar). I am quite active too. I run up the stairs barefooted many times a day because I just can't sit still.

Any ideas what this could be? Thank you.

Also, I have had my thyroid checked and the results were fine.