mc7

Yes, it totally could be an STD if your partner had any disease to give you. If you don't know, I would want to see a doctor or clinic. But if it can't be an STD, it's probably most likely yeast. People who don't really have any sort of sexual infection can sometimes still give other people yeast.

A simple little home remedy for yeast and other minor genital infections (that's always helped me) is cranberry juice. And I've found cream, milk and sweets tend to make it harder to overcome an infection or can even exacerbate it, so I avoid them until I'm clearly well again.

You especially don't want to get a yeast infection of your foreskin because that can actually make the skin grow tighter (potentially undoing stretching), besides which there are so many other ways infections are harmful.

If it develops into a real itch or pain or any other worse symptoms, see a doctor. Hopefully just with cranberry juice and water, it'll go away immediately.

Oh and if it's a (minor) skin infection like it sounds like it might be, washing and thoroughly drying the area is important.

And about your parenthetical question, yes you probably should now use your erect glans as the form to continue stretching your foreskin open until it's wide enough to retract easily even with an erection. But as you probably know, do not pull it back past the glans until that's easy and comfortable. Paraphimosis is worse than no fun. And finally, I'm not certain about this, I've never had phimosis to know, but I think that when the skin can move freely, you'll probably be able to desensitize the overly sensitive part from the way that you'll be able to masturbate then.