Although pre-medicating for dental work is a precaution many surgeons, and dentists, advise, it’s unlikely getting a filling without pre-medication would cause an infection in the knee. That you have pain in the knee, though, after a year, and that it’s constant and bothering you is something to watch.
Have you tried treating it with ice and elevation? It could be that something you never even noticed —a bad step, an unusual sleeping position for the knee, more activity than usual, or just nothing at all— has aggravated your knee and caused some swelling. Even when our knees are healed fully, they’re still implants and are surrounded by tissue that has had surgery, so can still act up on occasion. Try resting, icing, and elevating to see if your pain decreases.
I think it’s natural to fret about infection every time our knees act up. It comes with the territory, I’m afraid. Chances are it isn’t an infection, but if you’re worried, there’s nothing wrong with calling your OS and getting his or her take. Have you had, or are you scheduled for, your one year follow-up?