Water turning slightly reddish brown

Question

After boiling water it went from clear to brownish red. I boiled the clear water in a stainless steel pan and as it was boiling it started to become brownish red. The more I boiled it the more pronounced it became. I put the boiled water in a glass and compared it to the same water before boiling and it was clear. Just to further test if this was the water or the pan I used purified water and boiled it in the same pan and it was clear. What's going on? Why is my water brownish-red after boiling?