Natural birth control

Recently I've been trying to track my period and ovulation cycle with the app so I can time when it's safe to have sex and when it's not. I don't really want to get on birth control because I feel like it messes with my body too much and I'm too scared it will ruin my chances of getting pregnant later on. This last time however I used a plan b and my cycle was so weird it's like I got my period twice. Will my app be accurate on when I'm ovulating. Also.... if I already ovulated and I have sex, would I still be able to get pregnant even if there's no egg? Or how does that work exactly. If anyone has other good tips that have helped them not get pregnant by just tracking their cycle please let me know.