What causes nearsightedness to worsen.

I just came home from the eye doctor and according to her, my vision "is falling apart." I went from around -2.75 to -4.50. What caused my vision to worsen this much? Ever since I got a laptop, I've been using the computer ALOT. could that be a factor? and what do you suggest I do so that my vision doesn't worsen? and I'm 17.

It's controversial, but there certainly is evidence that close work causes nearsightedness. I also regularly took my glasses off when doing close work, and my myopia didn't increase much, and actually decreased once I got out of university.
At -4.50, that's too strong to read comfortably without glasses. Twist your eye doc's arm to write a prescription for computer glasses with a prescription of around -3.25, and reading glasses at around -2. You could also go with "computer bifocals" with the "distance" power of -3.25 and a reading addition (ADD) of +1.25

If you don't want your vision to get worse then DON'T wear your distance glasses if you are doing close work. I'm 19 and only use my glasses for distance- my prescription has stayed the same (-2.00) for over a year. You can try to improve your vision if you don't use glasses for near work, eating carrots and doing eye exercises.