Swimming is really good for you. Don't worry. Even if you are swimming when your period starts the flow is likely to be very light and probably won't be noticed. Have a tampon in your bag just in case. Sadness can be a part of PMS (premenstrual syndrome) which happens in the days leading up to a period starting or it can be more general sadness/depression brought about by the new hormones in puberty. Have a read at www.headspace.org.au or www.beyondblue.org.au. If you'd like to talk with a professional to help you deal with sadness try kids helpline (p. 1800 55 1800) or your school counsellor or GP.