In the long run, the purchase cost of the book Taking Charge of Your Fertility
http://www.amazon.com/Taking-Charge-.../dp/0060937645 may save you the cost of a lot of pregnancy tests.
It is possibly the best book on women's reproductive health. It should let you track exactly what is going on when with your cycle - useful for birth control, fertility planning, or for choosing the best days to use a pregnancy test. Read some of the reviews on Amazon and you'll see what I mean.
If you still aren't sure about the book after looking at it on Amazon, at least get it from the library, even if you need to get it through inter-library loan. Seriously, it IS that good, and it will give you a level of understanding about the details of every part of your cycle well beyond the info you find in any other source. Being able to truly recognize which day you actually ovulated (and it can vary from month to month) will save you a fortune on repeated pregnancy checks.
The cost of this book was some of the best money I've ever spent - which is serious praise coming from this tightwad!
Lynda