Please i need help im so paranoid?

gf missed her period 2 months ago. she is ireegular, she got it last month for 6 days, she had a few days of heavy flow. her period isnt due for around 10 more days but she has a lot of acne right now and is sometimes tired. what could this be? i had her take 10 hpt all negative over the course of two months so im ruling out pregnancy. could it be pcos or a hormonal problem. please help. we use dondoms except one for 10 seconds months ago, i didnt ejaculate and i pee'd beforehand so im ruling out pregnancy. what could it be? all the tests can back solid negatives