How do birth control pills work?

I've just had my first condom break ever. I'm on the pill, so I'm not worried about pregnancy, but a couple of days afterwards I had yeast infection-like symptoms — no unusual discharge, just itching. If I pick up some over-the-counter yeast infection medication and the symptoms go away, would I be safe to assume that was the problem? Or should I definitely go in for an STI check?

I've been on birth control pills for eight years and I'm thinking about trying a few months off them. They were prescribed when I was pretty young because of very irregular periods. I also was involved in a monogamous relationship for a number of years, so they were convenient. Now I have no steady partner, and am really curious to see how I feel without taking these pills. Would it be ok to go off them at the end of my pill pack or should I consult someone at health services first? If I go off for a few months and my periods are as horrendous as they were when I was younger, would it be safe to start up again on my own (I usually just see someone once per year for pill checks)?

I care for my girlfriend, whom I have been seeing for a few months now. I have no problem maintaining an erection when she performs oral sex on me. In fact I can climax twice in a relatively short time. We have, however, been unable to actually engage in intercourse, since when we go to put on the condom, I lose my erection. I'm not really having too much of a problem getting erect (i.e., I get an erection, and may lose it early on, but I am able to get and maintain another one for oral sex). What could be the problem? Stress? A lack of physical attraction? I will be grateful for any answer you can provide me.