Hmmmm....
If you think that you could be pregnant, just be aware that it might have happened/been conceived right before your period was due (your light one). When you are on the pill, your ovulation time is not going to be like that of a woman not on the pill.
It is very possible that if you are, that you could have conceived at that time, or honestly, even right after your light period, if you were sexually active then. If the test is negative, then if you ARE pregnant, it means that you do not yet have enough of the hormones in place to show a positive test.
This happened to me with my youngest. I KNEW I was pregnant from the time of conception...but I had to take a test every few days, for about 3 weeks, before it came up positive.
Did you take the test over the weekend? If you did, then wait about 2-3 days from now, and take another one. Take it first thing in the morning, with your first morning's urine. It contains the strongest amount of hormones, since it usually sits in your body longer while you are sleeping. If the hormone is there, and it is going to show up-morning urine will be the first place. After that, if it still comes up negative, wait another 2-3 days.
I also wanted you to be aware that I "spotted" with two of my children in the first trimester, around the time that my period would have normally been there. It wasn't a period...it was a couple days of spotting. This can be pretty common, and some women think that they are not pregnant because of it.
So basically...how long this is going to take to figure out, is going to depend on the conception date (if you are).
I would try a test every 2-3 days...until you get a positive, or until about 2 weeks from now. If there isn't a positive by then, then you are probably not.

To save money on tests, they have cheaper ones at Dollar General, Family Dollar, and other dollar type stores. (They are all the same-you don't need an expensive one.)