SOMEONE HELP! are my dates wrong?

Hannah • First pregnancy- miscarriage, 22.10.16💞👶🏼 Second pregnancy- baby girl, Maci 29.07.17🌼💫 Third pregnancy, baba bolton #2 due 24.12.18🌹✨
So according to every <a href="">pregnancy app</a> I have and the doctors and according to my last period, I should be 11 weeks and 4 days pregnant, I had slight pain in my stomach over the weekend so booked in to get checked yesterday, at the scan the midwife asked if my dates are right? I assumed so! However she said that from the size of the baby I shouldn't only be 7 weeks me and my boyfriend tried to work this out but the only way it could be right is if we conceived the same week I got a positive test, is this possible? I thought it took 10 days or so to get a positive test? But regardless of conception date wouldn't they still date the baby from the first day of your last period I'm so confused and scared incase my dates are wrong and the baby just stopped growing! Can someone give me some advice please?😩