Right knee pain while running

Hello everyone,
I am new to running again as I use to run 5k's err.. 9 years ago before life, job's, wife, ex-wife all that good stuff caught up with me. My question is since im starting back to run ive been sticking with a fairly easy mile run 3 days a week because i didnt want to push the issue and possibly get hurt as I have had knee problems in the past. I was running in a 5k and found myself caught up in a group of about 20+ runners when a kid running beside me got tangled up and fell onto me causing me to hit the pavement pretty hard. i ended up in the emergency room with 7 stitches to me left knee. Now the weird part... since starting back running i figured my left knee would give me problem's and have found that my RIGHT knee is the one bothering me?? it only happens at the start of a run and it usually last about 10 to 20 steps into the run. I stretch 10-15 minutes before hand and then walk a good half a mile to loosen up before i take the first step in the run. I would generally disregard the problem but it is quite painful when i first start the run and then it just die's off and does not hurt or act sore during or after the run? are there any kind of stretches i could use to loosen my knee up? or will more miles per week eventually ease the knee pain off? Overall i feel healthy and comfortable in the run but this knee pain at the start of the run has me wondering if i am doing something wrong. Thanks for all your advise in advance.
chris