Pregnant?

I was sent home from work today as had severe cramps and sickness. I had a nap and then at 6pm I decided to do a test just in case however I thought it might be way to early as I thought I had my period at the beginning of December however it lasted a shorter time of only 4ish days. Anyway....first test came back positive immediately and then did a second and then went and bought a clear blue test. I'm just confused why it says 3 weeks when I thought I had a period the other week ago?