What Causes Light Bleeding With Small Clots And Headaches?

I normally have a very normal period it starts out light with a brown discharge and gets heavier then lighter at the end it normally lasts 6 days. However I had my period starting on the 9th of this month and it was a very light redish pink. It lasted only 4 days(9th-12th) with just brown discharge on the fifth(13th). It was very light in flow and a bright red, with very small clots that were almost unnoticeable. I have been very tired, with mild headaches and my breast feel heavy, mildly painful almost. I bought a pregnancy test, Clearblue advanced digital. Which also gives the estimated weeks along. My problem is i don t know when i should take it for the most accurate result to know if i possibly am or am not pregnant.