Rash on belly.

My 15 month old pit bull/great dane mix has recently started developing a "bruise" on his abdomen. It's centered where his navel is. I first noticed it 3 weeks ago. It's been steadily growing larger, but it's been growing faster over the last few days. It's red, not purple or blue. I actually took a picture and loaded it on Photobucket at the link here:
http://i46.photobucket.com/albums/f129/mswanso502/15mosbelly.jpg
Just a little more info:
-He's about 70 lbs
-He's neutered (at 5 mos old)
-We were told his back left knee joints were loose when he was 10 weeks old-he had a steady limp.
-About 3 months ago, he started limping on his right front leg. The doctor did xrays and couldn't find anything, so he's been taking Ultram as needed; maybe 3 times/week or so.
-Every once in a while, I hear him cry out in pain. I'm not sure what's hurting him though. He's always laying down when he does it, so I don't think it's his legs.
-The bruise on his stomach doesn't seem to bother him. He still runs and plays with our other 15 month old male dog. If I press on it, it does seem a little uncomfortable. He did growl at me one time when I touched it. That's very out of character for him.
Any ideas what this could be?

Thank you for your response. I have also been talking to some people on a forum. They also mentioned skin infection. I will say that the rash is not raised at all. It all appears to be coming from underneath the skin. Today it's a little smaller and turning purplish right above it. Here's a picture from a few minutes ago:
http://i46.photobucket.com/albums/f129/mswanso502/15mosbelly3-1.jpg
I started giving him Doxyclycline (that I already had) tonight. I wanted to do something until I could get him to see the vet. Someone else mentioned a Lyme disease rash. I'm worried that it could be the problem since he's also been limping for unknown reasons. I have found several ticks on my other dog recently. I removed them, including the head and cleaned the area. I hope that's the right way to do it. I live in NW Louisiana. I'm pretty sure we're in a high deer tick area. Does it look like this could be Lyme Disease in your opinion? I always thought it to be a more round rash?

Thank you very much. I'm going to call and make an appt when they open in the morning. Hopefully it's nothing serious. I'll get him some new medicine. I didn't give him any today, just in case.

Ok, thank you very much!