Help with positives


Period was due yesterday and hadn’t came by 2:30pm so I took a pregnancy test and it lit up immediately. My last period was strange on dec 28th as it only lasted 3 days instead of 5. If I use the first date of that period then I’m only about 3 weeks. This seems strange to get such a strong positive at only 3 weeks on the day my period was due. Could I have been pregnant on the 28th and I’m actually further along? I have a dr appt on feb 7