Sex 7 days before expected Period and gf had Period on time. When should she take a blood test to be sure that wasn't İmplantation bleeding?

Pregnancy unlikely. A woman cannot get her normal regular period and be pregnant at the same time. However, she can get pregnant a week or two after her normal period. If she has some spotting or light bleeding that doesn't look like her normal regular period, she can check a urine pregnancy test about 3 weeks after sex (3 weeks after when conception might have happened) to see if she is pregnant.