Knee Pain - Help! :(

I'm sure this has been discussed before but I didn't have time to search the message boards. I have constant knee pain and it's killing me. It makes it hard for me to work out. I danced (ballet, tap, jazz, hip hop, you name it, I did it) for 18 years and as a result of that (dancing several hours a day, 6 days a week) it has taken a toll on my body. I have bad hips which don't bother me too much but it's my knees that really hurt me. I started running religiously last spring and since then have completed several 5K's, 10K's and even a Ragnar Relay Race. I did, however, pay for it afterward. My knees hurt after every single run or race. When winter rolled around, I started slacking on the running and am now trying to get back into shape. I haven't been running as much (because it's been hurting so bad) but have been walking on the treadmill during my lunch hour here at work at a 15% incline at about 3.5 MPH for 20-25 minutes and then doing strength training. My knees don't hurt during my work out but after they just constantly ache. They feel like they are swollen 2-3 times their size (although they're not.) I'm just not quite sure what to do. I don't want to give up up my cardio exercises because I really do love it. I can't take anything like ibuprofen because I have had stomach ulcers. I'm not overweight so I don't think it's the weight on my body; I'm 5'5" and about 130-135 pounds. (Depending on the day. ) Anybody have any suggestions to help with the pain?