Injured knee after two weeks in the gym and doing C25K

ninjakittens

Hello everyone.
Sorry for the long post and I appreciate anyone who can help!

I finally got back into a fitness plan (after only walking sporadically for a good while) and was really happy doing it and feeling great after two weeks when I somehow injured myself. I'm not sure what did it. I was doing the c25k program (running slowly) - I finished week 2, and was just using some of the weight machines in the gym while I was waiting for my copy of NROL4W to show up in the mail.

A week ago I did my C25k on the treadmill and was feeling good - then did the weights - still good. That night and the next day my knees were a little sore but I figured it was just my body getting used to exercising again. Friday I ran my c25k on grass - no weights - and by Friday night my knees were really sore. Saturday I walked about 2km to try loosen them up. Sunday I was hurting and Monday I was in real pain and couldn't go to the gym - same today (Tuesday) - it's really painful even just sitting down or walking slowly - and it seems to be getting worse not better.

The pain is mostly in my right knee - only a little in the left. It's on the inside lower part of the knee but is now spreading to the back and front - but the worst bit is inside.

I have good running shoes - I tried a couple of different pairs - my usual ones on the Wednesday and a more supportive pair on the Friday. I know I have a bit of an unstable foot when I walk - and high arches - so it's possible I under pronate. I'm not overweight - though could afford to lose 20lbs. 5'11" and 164 or so.

Any ideas on what I might have done, what I can do to help it or avoid in future? Can't really afford to go to the doctor now - or even if I could I can't afford physio if that's what they suggest! Student!