Time line help please !

Megan • 23 & Irish ☘️ 💍 29/09/2018 Baby girl born July 2019 after TTC for 13 months PCOS & thyroid dysfunction

So my last MP was 1 nov 2018. According to calculations I would be 12+3. I went for my 3 month scan and I seem to be only measuring 11+3. Does this mean I am only 11 weeks pregnant or is it possible I’m just having a smaller baby and I’m still 12 weeks 3 days?