I also started using the cheapie ones this month, and had exactly the same thing happen.

I actually tested twice a day (early pm and late pm), the day I got the strong line (not as stong as control line), was on a test I did late in the evening, and the next morning I woke with horrendous pains, and when I tested early PM - there was no line again - all happened very quickly!!

Get Bd'ing right away as it could only be hours before you O, and then its all over for another month.