I think we need a little Morse specific info in order to answer. 1st, are you M or F? And was the fungal infection in the genital area? As for the ‘stretch marks’ as you describe them, I don’t believe these are related to any type of STD, rather doubtful they relate to any type of sexual activity.
Glad to hear your fungal infection cleared up. I'm still doubtful these stretch marks have anything to do with sexual activity, unless you were in some extremely awkward position during sex that stretched the skin. And even then, a one-time incident like this seems unlikely to cause stretch marks. Possibly a Dr., maybe a Dermatologist, can tell you what it is, what might have caused it, and offer a treatment that will help...