You should count from the first day of your last period, even if it was irregular i.e. extra light, early or late. (I had irregular periods before my first pregnancy and that's how they counted and it was fairly accurate)
If they've been irregular as in farther apart it will most likely mean at your typical 8 week apt/ultrasound baby will measure more like 7 weeks. But most dr will still count from first day of your last cycle Until they do an ultrasound to confirm your due date.
I get bad symptoms from the day I get pregnant. Also sometimes irregulars period can cause pregnancy symptoms like sore/growing breast and stuff from the hormones being out of wake. So if you were feeling pregnant sooner that might be why as you did mention two months of irregular periods
I was leaning more close too 8weeks.. my symptoms are telling me I'm further than 4weeks but the last time my period came on was Dec.16