Clear blue digital test

Hi all, last Monday on the 5th of October I did a normal test which showed up fairly strong considering I was due period for 6 days, so I figured judging from the first day of my period on the 11th September I would be about 3 weeks and 3 days (last Monday) however I did a clear blue digital and it says 3+ which means I probably conceived before I thought I did! Now my last period was very strange and lasted 8 days and the day after it finished I had the fertile cervical mucas.  According to the results I should be 5 weeks plus not what I thought I was and that's 4 weeks 5 days.  Can anybody give some insight please