Light Period & Possible Pregnancy: Is a Test Accurate?

Question
I have always had regular periods and been on the pill for about 5 years now.  I admit not taking them always as I should but I've never been late like this time.  I got my period about 2 or three days later than usual and it's extremely light, and I'm thinking of taking a pregnancy test and was wondering if me being on my period might make it read any less accurate than normal.  Plus, before I noticed I was late getting my period, I took Bayer for headaches a couple of days in a row and a couple of diet pills while exercising and I'm wondering if that is enough to cause major prombelms and if so, what can I do to change that now?  

Answer
It sounds as if you may be pregnant.  you mentioned taking bayer for headaches...Headaches are one of the first signs of pregnancy you have.  Most people dont know they are pregnant by this time thinking it is just sinus, migraines or headcolds.  You should stop using the diet pills.  These can be harmful to you andyour baby if you are infact pregnant.