Does anyone know what i have done my knee cap popped out of place then went back in at football tonight?

i was at football it was raining and i turned on my knee and it popped out and went back in it was so painful for about 5 mins couple hours later it feels swollen and still hurts this has happend 3 times before on astro turf and once on 3g and am in pain for a bout a week before it goes then it just happens again please could someone help thanks.