Anyone encounter this before ? Is it normal?

So i went to GP to confirm my pregnancy and according to my first day of last menses, doc say i am at coming to 6weeks of pregnancy. I went back to polyclinic to get a referral letter cause GP can only refer to private and i wanted subsidise. About 1 week (i shld ardy be at my 7wks right?) later kkh call for appt, went for my first ultrasound and then see doctor. Doctor say i am not even 7wks, i am only about 5.5 wks. Doctor assured everything is fine cause got baby heartbeat ardy. Should i be concern? According to my calculations & the GP doc calculation is tally. Anyone experience this before?