Why does it stain when you sweat?

Is there any reason for sweat to stain skin and fabric PURPLE?

• I ran yesterday and when I came home and took a shower, I noticed that along the edges where my sports bra had been, it looked like a bruise. Then I noticed that the entire inside of my t-shirt was light gray-ish purple, especially dark under the armpits. Today, I did not run, but I noticed that under my dress, on my shoulders, armpits and a little bit on my back had turned purple, but more widespread, the entire area was purple, no just lines. The edges of my bra also turned purple. It did not come off completely from washing it, but did fade mostly. What is going on??? By the way, I run a lot (sweat a lot) and this has never happened before. Thank you :)

• Answer:

  Sounds like some dye from your shirt might have bled when it got wet with your sweat. Even if the shirt was not blue or purple, it might have had blue or purple dye. It's also possible the salts in your sweat changed the color of another dye.