Help plz

Hey so on the 28th found out I’m pregnant but when I went to the er to check they did a vaginal sonogram and results said I’m 5 weeks because they went by my last cycle but my bf didn’t arrive to the states till the 3rd baby making that night I must have literally been ovulating but the mean sac said 4.8 in size but that’s the size of like 5 weeks but I know I’m like 3 n change or 4 anyone else have had this and I haven’t been with any one else literally I don’t understand how that’s possible being I had a short cycle Nov from the 29 to the 1st was under lots of stress because he was crossing into the country I know it’s possible to have ovulated like the same day it ended or the next but how are the measurements that could it still be off because it’s early ? Does 5 weeks really mean I’m 3? This my third baby you’d think I’d be a pro but so confused because tomorrow would be a full month he has been here