antibiotics and the pill

So i've been on cerazette (however you spell it) for about 3 years and have not had a period ever while on it. Recently I started taking penicillin for tonsilitis and since then I have been having cramps and slight discharge. I have now started bleeding so I assume I am starting a period. Thing is it's the first for 3 years so it's kinda unexpected and I just want to check it's nothing wrong. I'm on the pill to stop periods so it's something that is obviously annoying for me and I don't want happening again.

I've heard that antibiotics can stop the pill being as effective. Would that include stopping it preventing periods?
And if so is there any way to prevent periods while taking antibiotics (I tend to get infections a lot so I can see this situation happening again)?

Antibiotics don't effect that particular pill as far as i know, i took it for a couple of years (and then 2 years in started having periods again and side effects so stopped taking it) and its pretty notorious for causing irregular bleeding in a 1/3 of people who take it so i would change to a different one really.

Hmm good to know. Didn't think it would change after this long. Think I might discuss it with my doctor if it keeps up. If it doesn't affect the pill I might just check the penicilin can't be doing any harm. Just seems odd it happened after starting it.

Thanks