What Causes Swollen And Painful Knees?

I am a 53 yr old female in good health. Within in last week have been experiencing knee swelling and pain in both knees. I have not had any trauma or injury. Nothing I can think of has changed. In April I was told by my boyfriend he had Syphilis - I immediately went and got tested it came back negative. A few months ago the health department contacted me and asked that I get tested again as they believe it may have been too early for the test to be accurate and wanted to make sure. I have been busy and just hadn t made time to got that yet. So now I m wondering if there is a correlation between the 2. I have had any symptoms but nonetheless am baffled at why all the sudden my knees are swelling and quite painful. I plan to go have the test done today.