weird knee pain

hi all,

about 3 weeks ago i tried to put on some cheap snow chains on the car. it meant i was kneeling in slush, on ice and generally struggling for 2 hours. since then i have weird pain in my knee. it seems ok and i think i have got over it then suddenly my knee will give way and the pain is excruciating for about 30 seconds to a minute. it then aches for a while and then it will go away then happen again and again. it doesn't hurt when i cycle at all.

edit: sorry forgot to explain where the pain is. it is behind the knee cap inside the knee. i can make it hurt by trying to over straighten the knee, eg when yawning, if you know what i mean?

on 19th december i had a cortizone injection in my elbow for an injury sustained in a rear end shunt in september.

i was told by the physio and doctor to take 1200mg of ibuprofen. i had an allergic reaction so i have stopped taking the pills approx 5 days ago. i had been taking them since the end of september


could you please tell me which knee support i need to buy.

thanks in advance