period help

I don't know if I should be worried or not... in September, August, and October my periods came, but only lasted 2-3 days and the only thing that came out was this brown liquid stuff and about a teaspoon of it came out in the 2-3 days (I don't know if any of that made sense). In November I had a normal preriod, but just in December it happened again.
I don't know if any body knows what this is, but if you do, that would be great because I'm a little worried.