1) The rhythm method is notoriously bad in terms of efficacy, even for women with regular cycles. I highly recommend something else. If you don't want the pill, some sort of barrier method might work for you. Plus I would recommend a condom no matter what - regardless of all promises of fidelity. This is your personal safety we're talking about. Plus it will help with your peace of mind in terms of preventing pregnancy.
2) If it concerns you, you can buy a home pregnancy test, follow the instructions and take it if you do not get your period. You're probably find but this might be useful for your own peace of mind.
And, yes, peace of mind - it might be a good time to reevaluate how using the rhythm method is likely stressing you.
A third thing, actually - contact your doctor. Ask about what you've asked about here, and talk about pregnancy and disease prevention. Believe me, your doctor has heard it all before. It is his/her job to advise you, and help you in this area. You are probably not pregnant but you might want to get it from an authority just what the odds are, depending on the methods you choose.