Due date help

I missed my period two days ago so I took two pregnancy tests and they were both positive. YAY! Now once I entered this into the nurture app, it asked for the date of my last period which was in October. The app is calculating me to be at around 5 weeks, but I had just started trying to conceive just two weeks ago. That was the first time that my husband and I had unprotected sex. So would I actually be two weeks pregnant? Thanks in advance!

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Basically,

We have had protected sex up until two weeks ago so how could I be pregnant five weeks? Isn’t it inaccuracte to calculate by my last period?