I worked for a Professor of Nursing as her nanny for several years and she was always amazed at womens reactions when they got pregnant while breast feeding.
If you are not taking birth control or abstaining then you are running the risk of becoming pregnant. It's just very simple.
Please consider the Nuva ring, it is a local form of birth control that inserts in the vagina and if wanted can be taken out for several hours for intercourse. It usually isn't felt though. It comes in the mail in a Styrofoam box and stays in the fridge until you put it in. Since it is a local form of BC it should not have any side effects.
If you are finished with having children then you might consider the Essure procedure. It is a small spring like thing inserted in the fallopian tubes and it has a substance on it to promote tissue growth to block the tube. To test to see if it is finished and fully blocked the doc runs dye through it and then no more BC ever.