I don't know your PT, and I don't know the situation with your tendonitis. So take everything I say with a grain of salt.
If your gut instinct is to find a different PT, I say go with your gut.
7 iontos in 2 weeks is rather unusual in my experience. Continuing when the patient is having an adverse reaction is also rather unusual. (It may prickle or tingle, but it should never sting and burn.)
How is your foot now? Giving your doc's office a call might still be a good idea.
And please don't get any more ionto treatments.
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson