First, I think you ARE brushing your teeth too often. I agree with the other poster that rinsing with warm water and Sea Salt is best after OP. Brushing 4-5 times a day is a lot of abrasiveness on both the gums and teeth. And I have no doubt that if you OP every day there's no need to brush any more than twice a day max - especially if you're using something like the Sonicare.
Gum disease is the bigger oral health issue by far than cavities in your adult years - and OP has no peer for prevention and healing of gum disease.
Secondly, OPing twice a day can cause some dental pain all by itself. I OP once a day, and my teeth usually feel sensitive to some degree aftewards. I think if you don't always stop after the 20 minutes (like myself) or don't always rinse thoroughly, the extracted toxins that are now in your mouth can cause some sensitivity. I'm ALWAYS more sensitive after pulls that exceed 20 minutes for example.
I don't doubt that if I pulled twice every day I'd have tooth sensitivity more often than not. Add to that brushing 5 times a day, and I couldn't imagine NOT having tooth pain.
I hate to admit it, but I brush once a day with the Sonicare and OP daily. But my gums and teeth have never been in better shape and I'm in my early 50's.