i have the same exact problem, first happened when I was playing basketball with my younger brother..it hurt soooooo bad, and it still continues to happen whenever it gets weak or i'm not careful with it...I went to the orthopedic about it and he said it wasn't too uncommon for girls my age(at the time i was 17ish), he said one of the tendons(i think) that keeps the knee cap in place was loosened by my injury and this is why it continues to sometimes pop out of place, he suggested doing different excercises to streghthen and support it such as squats and such but BE CAREFUL..dont over do it or it will be extremely sore...try not to lock your knees while youre standing around at your job, i cant myself doing it sometimes and it isn't good...other than that theres not much you can do other than corrective surgery if its THAT bad but...i cringe at the thought of surgery..if it keeps up you might wanna see the doctor but like i said there isn't much they can do for it, i was given a knee brace and sent to phy. therapy for a few weeks so i knew which excercises to do to streghthen it

Wow.. ouch.

You need to be icing your injury and taking meication to help the swelling.
It is possible you stretched or even tore something, however any sort of surgery will be optional. I opted to have surgery on my knee because I was too active not to. But in most cases you'll be fine as long as you get more muscle on your legs.

At least for awhile, try and talk to your manager so you're able to rest your knee during work. maybe by getting a stool or something to sit on. When you get done with work you need to do the whole RICE deal (Rest, Ice, Compress, Elevate).

You can also find a mini wrap for your knee, but only wear this when it's swelling. If you wear this for a long period of time then it will cause problems for gaining muscle.