help me understand breastfeeding and periods, please!

So I had stopped my progesterone only birth control at the beginning of the year.. I didn't have a period for the month after, I had one at the end of February and I was supposed to start again on March 26 and I haven't..so technically I'm 11 days late and I took a pregnancy test on Monday and it was negative..could it still be because I'm breastfeeding my son, or should i take another?? I'm so thrown because I used to be regular and right on schedule until after I gave birth to him..