Had Protected Intercourse, Feels Fatigue, Rashes On Neck. Syphilis And HIV Test Negative. Skin Diseases?

Hi i had protected intercourse with a new partner and soon after i have the following symptoms, xtreme fatique the next day and some rash in the neck, a flulike feeling n chills on the day after. on the third day, visible redness on the lips n around it which hurts when i was shaven, a kind of ulcer(feels like the mouth thicken) inside the lips and tip of tongue which burn when using mouthwash . i had syphilis n hiv tested about 5 weeks after the encounter which came out negative. Pls help whether this is std or a type of skin diseases as I never feel fit since after the infection and the feeling in the mouth, tongue, n the rash persists. Thank you.