Positive line still faint after 2 days

I got my first faint positive on Thursday (10dpo) and I've taken a test everyday (let's face it sometimes twice a day) since..
Today (Saturday) is the day I am suppose to get my period and it hasn't come, I took a test (Dollarstore) this morning and it was postive but it's still really faint.
​I know a line is a line but should I be worried my line is not getting darker.. It's been 2 days since my first and I thought your HCG doubles roughly every 2 days.
​Is this nothing to be worried about or is it something I should be monitoring..