What Causes Very Small Amount Of Blood During Periods?

I thought I came on my period on Monday, I had a small amount of blood so I put on a pad then went to sleep as it was around 11, the next morning I woke up I had no flow overnight the same happened I had nothing till today on the bus home from school I had a small amount of blood so I put a pad on and its been about 3 hours and I have had no blood, I usually had really heavy flows until a couple of months ago when I tried to use a tampon for the fist time ever since then my flow has always been quite slow, I am slightly concerned because I don't know what this means it has never happened before and I do not understand if it is my period or something different.