While many women do not get their period until after stopping BFing, there are those unlucky few that get it prior. My cousin's sister, which both her kids breastfed for well over a year but got her periods like clockwork when the babies were 3 months old.
My first period started at a year. I was still breastfeeding, but it did start the morning after sex (not the first time we had sex after the baby though). I bet you were about to start and the sex maybe brought it along.
I don't think you have anything to worry about. Sounds completely normal, though a bummer!