Knee question

Hello, back in early July I fell on the edge of an escalator stair. I recieved 8 stitches and it was a deep jagged cut. In the first couple of days I started a low grade infection. I was treated and it went away. But, since then my knee does not seem to be getting better. I have pain going up and down stairs, I cannot run, after walking for just a few minutes my knee feels very week and sometimes gives out a little on me. And recentley I have been getting an achy feeling in my knee even if I am not walking. I did not have a x-ray done and I was wondering if you would suggest getting and x-ray or MRI done. And what you think this may be. I appreciate this. Thank you.