I haven't seen too may be able to actually use the natural birth control effectivily and if your under 25 or over 40, you got a 50-50 chance, since your system is either still is trying to find that cycle or it's trying to end that cycle, and not all women release an egg with their period it can be days before or days after. Which can get you in trouble right there. If it were me, I'd be using other forms of birth control that are effective while your on antibiotics, like condums, the sponge, etc, go see your doctor as there are many many different types out there, that you can use with your antibotics, and are pretty effective, it's just the birth control that you take by mouth, patch, shot, or get implanted that the antibiotics screw up. So you got quite a few doors open.
But I would seriously would not be using the natural birth control method. My half sister used it, because her mother wouldn't allow her to have birth control, she ended up with 5 kids before the age of 21.
Also if they switch your birth control your at a higher change of getting pregent as well, for the first 2 months that you use it. A lot of times doctors don't tell you this, until you come in feeling sick and they tell you, your pregent. Then it's oh by the way you were suppose to be more careful for two months. They did this to my mother, who wasn't even suppose to be able to have kids. The doctor switch her to a different pill, and about a month later, she found out she was pregent.
Good luck,
Kit