Hi, I Have Had A Red Patch On The Roof

Hi, I have had a red patch on the roof of my mouth now for about a mouth.. It is very sore and burns so much sometimes. I was awake most the night last night having to drink sips on water to cool it down.. Iv been to doctor who suspected thrush but then wasn't 100% as no white patches in mouth she prescribed some gel which didn't really help.
Then I went to dentist who again wasn't sure what it was but told my to try some nystatin mouthwash stuff incase it was thrush..
Again tonight it's been so sore, wondered if you may have any idea?
Many thanks