My girlfriend's period is late by 10 days. A week after we had sex for the first time she said she felt dizzy sometimes and a little nausea. A week later she missed her period and is now 7 days late. She still has a little Nausea and dizziness sometimes. Could this be an STD or yiest infection? Pregnancy test after 10 days late was negative. I don't have any STD symptoms.