Yes, you can become pregnant, especially if you are not using any type of birth control. The "pull out" method is not reliable in any way, because during any sexual arousal a man's penis can emit pre-ejaculate fluid (or 'pre-***') which can contain sperm. In other words, you are taking a huge risk of becoming pregnant when you do not use birth control because he may possibly be leaving small traces of sperm inside you whether he realizes it or not. If you are ovulating and having sex without birth control, you are increasing your chances of pregnancy. The best thing to do is to speak w/ your doctor about your options for taking birth control. If you are sexually active you should practice safe sex all the time; use condoms.
But on the other hand, if you have successfully been avoiding getting pregnant for 5 years, I think what you are doing is working. If your boyfriend is your only partner, you are fine - enjoy, but it is risky, Clement has explained why perfectly