VCF for B/C?

Has anyone used over the counter VCF as a method of b/c and it fail? I am super sensitive to almost every form of B/C other than this....which seems to be working thus far. We aren't trying to conceive, as we have a 9 year old and an 8 month old currently. My cycles are very regular, but I am 4 days late.... I have been tracking my bbt just to see what's going on and it has climbed in the past few days. Today was 98.6 (the highest yet). I'm a little concerned, so I tested with a leftover HCG strip but it was negative. What's happening here!