Irregular period and fertile days

I have irregular periods. Sometimes I am normal with about 30 days between periods, and sometimes I can go 2-3 months. I am 18 years old and I just lost my virginity a couple months ago. How do I know if I am fertile or not when my periods are so irregular?! I don't want to have sex on those days, but I'm not sure when they would be then... I had unprotected sex on a day that glow said I was fertile (withdrawal method used) and my period is late- but possibly just irregular.. This is why I am slightly freaking out. I may have or may not have been fertile that day?!  Which increases chance of pregnancy even if no cum was ejaculated? Could precum and fertile days be a bad thing?