If you're on the pill and taking it correctly, it means that your body does not ovulate at all. The pill tricks your body into thinking you're already pregnant, and therefore doesn't release any eggs to be fertilized. Your period always comes when it should because it is a withdrawl bleed when your body isn't receiving the strongest hormones.
The chances of pregnancy are very low if you take the pill correctly and if a condom is used. There can be sperm in pre-ejaculate, but you should be protected with the pill. The 'pregnancy' symptoms you feel are most likely period symptoms, and your mind may be playing a tricks on you if you're really paranoid. I know what it's like to be paranoid...it's stressful and stress could possibly mess your cycle up, so try to relax and not worry so much. You're taking a lot of precautions and should be proud of yourself.