What Causes Yellow Sweating?

Hi there. I'm pretty much always warm - so much so that my friends used to call me a human radiator. That's not really the problem however. I am sweating quite a bit at the moment as the heat has finally hit the UK, and in wiping my face on a while paper towel, what was left on the towel afterwards was yellow. Now, I understand why sweat can stain white shirts yellow, but why would a tissue be yellow immediately after wiping my face?