I have had this problem for a number of years (now 22); after I have a shower I pretty much always have a sneezing fit/running nose for about half an hour. I'm wondering if I might be allergic to the shower gell, shampoo or maybe the towel? I also often get a red rash on my body if I have the water a bit too hot (often happens in the winter when I'm cold and adjust the water temp to compensate!) it normally goes after about an hour or sooner.
At the moment I'm sneezing more than usual (throughout the day - not just after my shower) but I think this may be hayfever considering the time of year. I'm having a few other health issues but these are unrelated to the sneezing after showeing.