What is going on with me?

Okay so let me start from the beginning first... before I had my baby 6 months ago my periods were very irregular, after I had him my periods were right on track, then this last month I missed my period. I was supposed to get it on the 16th and it is still a no show. I have been super moody this last month, my boobs have been sore here and there but I have no other symptoms. I took a pregnancy test a week ago and it was negative. Does anyone know why I'm feeling this way? I never got like this until this month...