My gf missed 5 days of her birth control this month she has taken two prenancy tests both negative but now 6 days late on period any ideas?

Do another test: Repeat another pregnancy test. Stress, excess physical activity may lead to menstrual cycle delay, but pregnancy would be the most common, and obvious cause

Pregnancy: She should have a laboratory pregnancy test done. If she is having difficulty with birth control pill compliance, perhaps she should see her primary care provider or a gynecologist to explore other birth control options such as Implanon, the IUD or the depoprovera shot,