My mum won't stop stalking me even though I'm 20yo

Your mum sounds a real diamond - many would swop and pay you a cash balance...

Sounds like she has a chip on her shoulder about drinking and maybe she has something in her past that has made her like that - maybe she got hurt or something to do with alcohol?

You should ask her about it but I doubt she will tell you - might not even remember what it was herself...

Maybe this is made worse by the empty nest syndrome?
Maybe she hasn't many friends and no hobbies?

Make no mistake; at 20 you are far from grown up but you do have the right to make your own mistakes...

It is your mum's job to advise you and to pick up the pieces if things go wrong but not to follow you around even if all she wants is to keep you safe...
Obviously, while you are living with her it is good manners to abide by her rules as far as possible but it might moderate her behaviour and those rules if you make it plain you simply won't come back after college or you will be out of her life forever as soon as you get the chance!

How about challenging her to show you how to have a good time without a beer or two...
Can you fix her up and go out as a party or a foursome?

Being paranoid doesn't mean they aren't out to get you. I suggest you don't underrate whatever premonition made your mum so protective - I could easily have been dead as a 16 year old if it wasn't for my mum's premonition!