Help! Please :)

Ka

Hey guys. So my last period was 12/15/17. My cycles are 21 days. My expected period was to start 1/9/18 and it never did, so I took a test that day and got a faint positive. Fast forward to this week and the test line is darker than the control line. My question is, how far along would that make me since my cycles are only 21 days? Thanks :)