I have been getting rashes on my neck?

Okay so last week on Sunday i found a rash on my neck so i put on some lotion and sure enough it went away a few days later. this morning i woke up with a prickling rash on my neck again. this time its more in the front of my neck. there are blotches that are painful when touched. i have been using hydrocortizone and it seems to be working a little. any idea why this is happening? oh, also it has been very hot and i have been outside alot as well. i am on the swim team too and the clorine level is pretty strong