positive opk but negative hpt

So my period is due either the 15th or 16th of this month. I took an opk on the off chance on Friday and it came up with two lines. Thought this was a bit weird seeing as I've already ovulated a week and half ago! 
I ran out of hpt (waiting on another eBay order this week) I went out and bought a first response and tested Friday and nothing then tested this morning again and still nothing but my opk are coming out with two lines still? 
The only other signs I've been having is on / off crampy pains. 
Has this happened to anyone else? What was your outcome.