Please help I'm driving myself mad

I usually get my period every 30-32 days. Today is day 36. Yesterday I ahead a drop of brown blood in my underwear and today the same thing, only a drop. 
This is very unusual for me. I have been feeling tired lately and took a test on day 31 which came back negitive. 
​Is this just a very very light period or is there a chance I'm pregnant  ??? Anyone else experience this before ??