Take a test. But it could just be how your body is. It takes awhile to adjust to not being pregnant and get back into it's own cycle.
Condoms used with spermicide (you can get the foam kind or the pills you insert, etc) work really well together and gives you the added protection of .T.W.O contraceptives.
You may want to get on some long term birth control if you don't see yourself with more children. An iud is a good method and you never have to worry about being protected.
Good luck!