Your mouth ulcers at 4 days post exposure are not HIV related. Never take someone's word that they are negative. If you had unprotected anal or vaginal test then you need to test.
I totally doubt it. She said she was HIV negative why don't you believe her? Since you are in touch with her you could always ask to go test with her if you are concerned but your symptoms have nothin to do with HIV. Your risk was one in 1000 for vaginal sex WITH AN INFECTED PARTNER your partner likely was not hence no risk. Good luck, I wish you all the best.