I don’t know your full medical and sexual history so I am unable to give specific information about what could be happening, but I can give you some general information based on what you have written.
It is possible that an STI could have been passed to you when you had sex 3 weeks ago. STI can cause irritation to the vagina that may cause itching, but it is also common to get conditions like a yeast infection that can cause itching as well.
Some common symptoms for being pregnant is a missed period, swollen tender breasts, feeling sick, loss of energy, going to the toilet more often. Given what you have described it does not sound like you have any of these symptoms. Using birth control would really reduce the chance of you being pregnant.
I would recommend going to see a health care professional and describing your symptoms to them. They would be able to do some testing for you and let you know what could be causing the rash and vaginal itching.
If your period does not come this week you could do a pregnancy test next week.
Let us know if you need help finding a clinic.
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