What Causes Severe Burning Sensation In Skin?

My skin burns, it feels like a bad sun burn but you can t see anything. My last one was under my left armpit, it lasted 6 days. 8 days ago I started having feeling like I had a UTI and went to see my doctor today. I can t sit, it feel.s like I am on fire, I am tired and feel like am coming down with the flu. My PH was checked and it s a 9. She can t figure out what s wrong. I do not have a UTI but she found that everything was bright red inside, no rash. Please reply and thank you.