Should i be that worried ??

My baby was born may 18th 2018, i got my first period august 18th and since that day i havent got one , im currently breastfeeding but my periods have always been regular with a 28 day cycle.. today So far I’ve taken a test and it’s negative... any advice or ideas why I havent got my period.. my stomach just been feeling sick lately, like everytime I eat something it takes me to the bathroom or i get that sick feeling in my stomach...but i dont feel bloated or i havent got that brown spotting when you have missed your period.