breastfeeding and your cycle

I know your cycle can get messed up if you breastfeed exclusively(which I am) and I don't want birth control because if I get pregnant great if not oh well. My cycle was normal until 3 months and now it's 4 1/2 and nothing. I feel like I need to test every week just so I can know if I am pregnant. I guess my question is do I need to test that often or when do I need to?