When i masturbate, i break out into hives. could i be allergic to my sweat?

Define hives: Hard question to answer because it depends on what you are calling "hives". This is typically what we medically call, hives: http://wb.md/2pd3zIv In terms of allergic reactions, we don't have true allergic reactions to substances that our own body produces, otherwise, you wouldn't have survived through conception. There are autoimmune diseases, but those are mediated by different mechanisms.