What Causes Inflammation On The Surgical Scar Post Knee Replacement Surgery?

I have a fairly new scar on my knee from a knee replqcement that was done in Nov. It healed nicely however I ve noticed over the passed couple days part of the scar has become inflamed and the knee has become pretty tight underneath where the inflammation is. I was wondering what could cause that. I had a couple teeth pulled on thurs and it was around Sunday that I noticed the changes.