If you get bad heatrashes you can try supplementing with beta-carotene, which converts to vitamin A in the body. But consult with someone in a health food shop first if you do get it, because you need to get the dosage right. Too much can be toxic, and if you're a smoker it can actually promote cancer so you shouldn't take it at all. So don't just go and get it without getting proper advice first.
As for bedsheets... are you sure it isn't the washing powder you're using? Try going for a natural one, like Ecover, or even better these: http://www.soapods.com which are cheap, organic, and completely chemical and allergen-free (as they grow on trees). I use them, have done for years, and although I was skeptical at first they work better than anything I've ever used, they leave things really soft, and you don't get allergic rashes and eczema. Lots of health food shops will have them, or you can order online from lots of places.
What kinds of things do you eat? Need more detail again if that's not enough to start with (you can PM me if you want). 
_________________
We are a fever, we are a fever, we ain't born typical...