Irregular period?

So after giving birth i got my period back 40 days later i had a normal 7 day period i was due for my 2nd period 4 days ago nd still nothing im not on any birth control and have had sex however i did a preg test and it looks negative to me is this normal after giving birth and breast feeding? My baby is over 2 months old now is there a possiblity that i could be pregnant but its just coming back as negative or maybe that my breastfeeding has just madd my period irregular??