Is this correct?

My last period was dec 6th-10th 
My next period was supposed to start on the 12th, in 2 days. 
Well yesterday morning I got a faint positive pregnancy test. So I did the <a href="">due date calculator</a>, and it says I'm 5 weeks today. 
Does this sound right??? 
I'm afraid my tests are too light, being 5 weeks I thought they'd be darker...
*UPDATE* after depo, my cycles are 37 days long, not 28.. I'm actually 3w6d... I configured it to my 37 day cycle, not 28, that's why it was telling me I was 5 weeks..