Dont panic! If you don't have overnight pads, either put two pads one over the other, or get up at midnight to change pads. As far as swimming goes, if it's for school, discreetly talk either to the school nurse, or your gym teacher and they should excuse you. If not, most periods stop in the water, however if you have a heavy flow you might be best making an excuse such as 'I have an ear infection' or 'I'm to tired' hope this helps!