Well normally what happens is your period kind of stops but then when you come out of the water it will start again , it doesnt meen that you have to wait another week for your period to finish but if you did bleed a bit in the water and it showed I guess you would kind of be embarrassed but you could try a tampon, not that I have and I'm 13 , because it looks painful lol x but you could try it. Also if you went swimming with no tampon it might not be very hygienic , I'm not sure though. Or you could pass on swimming when your on your period.
