Gaining

I’ve always had a fast metabolism and found it difficult to gain weight but recently I’ve begun to notice a big change in my energy levels. Even though I haven’t gained much weight, I feel tired and hungry a lot more.

Is my metabolism finally starting to slow down?

Have you been moving less lately?
Your cardio will change faster and more drastically, your metabolism is a different animal. As soon as I ignore (or can't do) cardio or walking for a while, my energy levels plummet as well.

It's more likely you are deficient in something. Have you gotten any sun lately? What's your diet like? Do you take vitamins?