I used to have a slow(ish) metabolism, but now it's really fast? Is this healthy?

It is normal for your body to raise your metabolism when you are exercising. I was 6'0 and 164 pounds when I graduated HS, and I gained 30 pounds of muscle after boot camp. I ate everything in sight, but I was active. You are at a healthy weight, and keep eating, drinking water, and exercising. Maybe just maybe you might even get noticed by that person you had your eye on for the last year.