Why has my metabolism slowed down?

How old are you and are you male or female?

@chyna I am 21 and am female.

That is about the time my metabolism slowed down also. Your body changes as you get older and if you are in college, it seems to be worse. Perhaps from partying and eating too much junk food.

It may be you were simply more active when younger and a busy schedule has made you lazy. So your metabolism is getting sluggish. Its never a bad idea to check your thyroids.
Also your eating habits may have changed as well.
Are you eating more in the evening before bed? Eating more starches and sweets?
Also about now your hormones should be settling from puberty so its possible it is what your metabolism is meant to be now that your body is more stable and you also are no longer growing.

Oh I forgot to mention that—I have definitely been drinking a lot more/partying, staying up late – even on the weekdays. My eating habits have not changed though.

Even light beer can have 110 calories in it. That is an easy 600 calories if you drink a six pack.
It it didn’t pack weight on people than the phrase, beer belly wouldn’t exist.
Some even go over 200 calories.
Lots of carbs in beer as well.
Youth and fast metabolism isn’t a life time guarantee. Only death can last forever.
The late nights may be also forcing your body to hold on to anything that will povide long term energy. Its been proven that lack of proper sleep will lead to weight gain. You body need adequate time to repair itself and drinking and not sleeping is not helping it to function at 100 percent.