A bit fed up

Today my husband and I went for an early scan and we were really excited to hear our heartbeat for the first time. According to our app and calculations I was 8+3 . Got there and had the scan which all went well but only to be told from measurements I am only 5+5! I'm so cross because I feel like I've had three weeks robbed from me! I'm so desperate to get to 12 weeks so I can tell my mum. On the plus side we were told everything looks good and they were pleased. Is it wrong I'm fed up?