I have not had a period since February (I am not pregnant)

I have a 19 month old baby that I'm currently nursing. I know it's common not to have a period While nursing but my period returnd when my LO was 5 months old (way earlier than it did with my other children) and was regular for 8 months. Then, 4 months ago my periods just stopped again. I guess my question is could this still be due to breastfeeding or could something else be going on. I have taken a few pregnancy test all have been negative. I'm not on any birth control but I was at the time my period originally returned. I plan on giving it one more month then making a doctor appointment,

but Has any other breastfeeding mothers had their periods return only to go away again?