Thank you for your question. There are many different possible answers, and you will need to speak with your doctor
if these symptoms are persisting. The fact that you suggest that your symptoms all started close to each other after a single episode of swimming suggests that perhaps this is some sort of reaction to something that you have done. While there are obviously chemicals that are used to treat pools and keep them clean, which you may have reacted to, there is also the fact that usually people are getting more sun when they are swimming. This can result in sunburns in any body part or skin that is exposed for too much time without enough protection. In cases of sunburn of the scalp, it is common for there to be extra flaking of the skin of the scalp that can have the appearance of dandruff. At times this can bunch up as well and result in lumps. There are also other reasons for lumps after swimming and increased environmental exposure, most of which should resolve quickly and with good care. As there are some other things that are more serious and permanent that could also cause this, please speak with your doctor.