Ive had knee surgery 3 1/2 yrs ago that never healed, im on antibiotics for a gum infection, now my knee feels better, was my knee infected as well?

I don't think so: certainly not for that long a period of time…otherwise by now, I would think the issue would be much worse. glad your knee is better though.

Knee pain: Some antibiotics have antinflamatory properties( tetracycline) . Since it is not uncommon for post op knees to have sustained some damage to the articular surface, one may feel better after treating with a course of antibiotics.