I had very irregular periods so had an ultrasound dating me at 7 weeks pregnant. does this mean i conceived 7 weeks ago or 5? and could the ultrasound dates be wrong?

More likely 5: Pregnancies are dated from the first day of the last menstrual period, and when that date is unknown an ultrasound is the most reliable way to date a pregnancy. Even when dated by ultrasound though, the date assigned is the estimated date of conception + 2 weeks (so in your case, conception likely occurred 5 weeks ago). First trimester ultrasound is reliable to within about 5 days for dating.