What Causes White Spots All Over The Back, Chest, Shoulders And Arms?

Hi there, my problem is I keep getting loads of white spots all over my back, chest, shoulders and arms. It s blotchy like hyper pigmentation or something. I do go swimming twice a week and was wondering if it was something to do with that? I have only noticed it since I started swimming but it keeps on getting worse! Do you know what this is and what I can do to prevent it? Thanks