I Have A Swollen Knee, That Is Red, With Raised

I have a swollen knee, that is red, with raised red blotches and it is moving up. I have a scar from a previous knee surgery and the scar is even red. This is my 3rd week of having this. It is not getting better. It is throbbing and hard to bend, probably because of the fluid.
I am a 46 year old female. Healthy, but I am on doxycycline for bronchitis that I will be finished with tomorrow.