I actually had this happen to me last month. I was getting 2 lines (not as dark as the control) for several days then they got lighter, I was thinking what the hell is going on, I had heaps of I.C opks so I kept testing and a few days later got a positive. If you have some left I would recommend keep testing. Do you take your temp each day? If so you should be able to tell by your temp chart if you have O'd or not.

Hope this helps and good luck!!!