It is normal for your cyles to be erratic when you start menstruating again after pregnancy. If you are breastfeeding, your return to menstruation is delayed, and again, it can be erratic at first before your body settles into normal cycles. Having said that, it is very possible to get pregnant while breastfeeding before ever having a period. Having a period is the obvious sign that you have started cycling again, but it comes after you have already ovulated. Since you did already have a period, you know that your body is cycling again, so you are able to concieve now. I would take a test to make sure you aren't pregnant.
There is a wonderful book out there called "Taking Charge of Your Fertility" by Toni Weschler. It explains things that I wish I would have been taught when I first started menstruating as a teenager. Anyway, it also presents the fertility awareness method of birth control, which is equivlanet to Natural Family Planning. So if you are against contraception, you should get this book. It will help you learn your cycle and to know when your are or are not fertile.
Anyway, I would take the test. Good luck.