Isn't there doctor patient confidentiality where you live? Even if the country is strict, can the doctor start spreading the fact that you had sex?
Yeast infections are pretty common in women. If you take an antibiotic, it can sometimes cause a yeast infection by getting rid of all the normal flora in the vagina.
I don't think your period has anything to do with what is going on. You were just early for some reason. However if early becomes the norm, then you need to see your OB.