What causes you to have a flare up?

So, I was getting some physical therapy for a bad knee and it threw me into the worst flare!!! It was so bad, I had to stop the PT.

I honestly don't know at what point to stop myself when it comes to exercise. I was handling it so well and all of the sudden, BAM.

Anybody else have this problem with exercise induced flare with their inflammatory arthritis? Thanks!