After a month of no sex me n gf we have sex but after 15 o 20 mins condom has a little blood from her why?? period in 2 weeks

Blood: it could be some midcycle spotting from the lining of her uterus, but it could also be a small tear inside the vagina or bleeding from an inflammed cervix. Not an uncommon thing to happen.

Doubt any harm...: Intercourse involves a lot of movement and friction in a very delicate area of the body. Its not unusual for this to result in a small amount of bloody discharge. Unless it was painful, it probably doesn't mean any harm was done. Also, ovulation happens midcycle and can sometimes result in some spotting. Best wishes!