Hello and welcome to HCM.
Thank you for your query.
There could have been an irritation of your nipple or
areola region, as swimming pool water is treated with chemicals like chlorine.
This could have resulted in an irritation of the region, as the areola and nipple regions are very sensitive. I would advise you to keep the area clean and dry, do not wear a tight brassiere for a while, to avoid friction, and use a mild moisturizing lotion 2-3 times a day; avoid using strong soaps in the region, or strong detergents on the clothes you will be wearing for a few days.
If your symptoms do not subside, or worsen in the next 3-4 days, then you will have to visit a doctor. You could also apply warm compresses to the region to promote healing.
I hope I have succeeded in providing the information you were looking for. Please feel free to write back to me for any further clarifications at: http://doctor.healthcaremagic.com/doctors/dr-shoaib-khan/64581 . I would gladly help you.
Best wishes.