This sounds like cystic acne. This is very common at your age. I'm a 19 year old female, and i have had cystic acne, and it is slowly going away as i get older.
Cystic acne is hormonal, so it's normal. But one thing you don't want to do is pop them when they are not ready. This will break little blood vessels in your skin and cause bleeding under your skin that will become a bruise.
Only pop them when you see a white head that looks like it could at any time :P (I know it's hard, but if you keep messing with it, it will only get worse)
They can sometimes last for almost a month and they are very annoying. And because they create such a deep crater under your skin, it is possible for it to come back over and over again. But no worries.
What you should probably do is put a dab of benzyl peroxide on it. If you don't have very sensitive skin, i would suggest using 10% BP because this will make it disappear faster. If you apply it right after cleaning your face before bed, you should notice the cyst has significantly decreased in size, and perhaps scabbed over. DO NOT PICK IT lol. Only pick the scab if it is practically peeling off. You don't want to force it. This should really help you out.
Being stressed does cause a lot of hormone changes as well, so this could be contributing. So just hang in there bud you got this!
- Take care! <3