I had protected sex 3 days ago with my boyfriend and my vagina has been slightly irritated since yesterday. what could this be?

Yeast infection?: Yeast infection is the most common cause of itching or irritation of the vaginal area. It's not caught from partners, but symptoms can be first noticed after physical contact like sex. Also allergy or chemical irritation, e.g. from latex (condom), spermicide, etc. If ores or the irritation becomes more severe, herpes might be a consideration. See a doctor for diagnosis and treatment. Good luck!