J.J.
It could just be your body getting back to normal and your period is about to start. However, it is entirely possibly (though rare) to get pregnant when you aren't having periods. It happened to me with my #1. I hadn't seen AF in about 6 months when all or a sudden I found out I was pregnant!
Either way, a baby is always a blessing. It may be a difficult time, but think about how wonderful your first two are. Congratulations!