How regular is your post partum period?

I had a baby 7 1/2 months ago, and even though I breastfeed, I got my period in February, and have continued to get it each month, pretty regularly, usually about every 30 days. Now I haven't gotten it for 36 days. I took a pregnancy test that came out negative, and I don't feel pregnant. Before I was ever pregnant with my first, I could bet money on the day I would get it, because it was so regular. What's going on with my body??