WHY DOSE IT HURT AND WHY IS IT WHITE PLZ PLZ HELP IM SO SCARED!!!?

OK plz help me im a 13 year old girl and i woke up on friday my inside lip like how my lips is facing the tongue and teeth well i saw that it was sore and kinda red and it hurt so i brushed my teeth and it kinda felt better so i went to my friends house and she like had the same thing i did but not where my red cut looken mark was hers were more nere her cheek and now we were like thats wierd its now starutday i look at my cut now it has like white on it and its really red hers were like mine so next day sunday today i look at it its brite red around the mark and it has white on the cut thing like last time and it still hurts when i put my tongue on it and now i was in the car and my friend call me and she asked her grandma wat it was she said it was a diseas thing when u kiss a boy that dose like drugs? and my bf douse sometimes douse weed but he told me he dident when we kissed and we havent kissed in like 2 weeks and my friends bf did smoke weed and she also told me that this disease can kill u and im soo scared and im cryen but her grandma send to brush my teeth alot and to gargle salt water thats warm PLEASE HELP ME I DONT WANT TO DIE IM TOO YOUNG IM ONLY 13 PLEASE HELP ME