Brown Period with some clog in it? i'm only having some blood on the pad

My period usually start at the end of the month. I had protected sex (only 1 sex partner) on march, and i had my period on 31march. i then missed my period on april. my period came late on 14th of may. however, it is like brown in colour with some clog in it. i thought that it might be the leftover discharge from my pervious period. however, today is the 4th day of my period, and there is still no red blood in sight. just very minimal brown clog and blood on the pad. 

i also did a pregnancy test just incase. and no, i am not pregnant. 

anyone know what is the problem ? i had never have brown period before. other then the start of period or ending of period. this is my first time having 4 day of brown period. i am worried .