knee pain

I just started running more seriously a few months ago and I have some knee pain that I don’t know how to handle. I ran a 10k last week on Sunday as a longer run and then a 5k a few days after. The next day I got some pinching in the back of my knee that gets a lot worse after walking a long time and especially when using stairs. Are there any exercises I can do to strengthen it or help it heal? I haven’t been running, but I live in a walk up so stairs are pretty unavoidable and it’s super painful. Thank you all