Can you help with my period?

I am having trouble with my period. It's normally a regular period comes around the 5th of the month faithfully. Last month it came on the 11th. this month on the 15th can down for 1/2 day and went away. one the 18th start to come down light but it's so light i don't even have to wear a tmapon if i choose. this has never happen beofre. I missed a period one month only once and that was over 10 years ago. My doctor did a pregnancy test negative. And I am not stressed out. what could be happening. oh by the was the blood is a darker brown. sorry for so much detail. please help