Same exercise, Different Results?
 

I know this question is rather personal, as in specific to myself. But I'm wondering if anyone has had a similar experience, and had a reason as to why this happens.

I've always been relatively chubby my whole life - about a size 10. Then when I was 19, I started running on the treadmill for 60 min a day. No change in diet, just walking a lot more.

Then within 6 months or so, I was down to a size 4-6.

4 years later, I gained back the weight, and tried to do the same exercise routine, perhaps even more exercise than before. NO RESULTS, maybe down one size.

I know that perhaps my metabolism slowed down, but I'm 24. Can it slow down that much? I had my blood tested, and it's all normal. And I know no one is a replacement for doctoral advice, but any educated guesses?