Bubble/blister on brow

[Susie snip snip]

hi, tonight I waxed a friends brows as I have many times, but after about 15/20 mins a bubble/blister appeared... the wax was defo not too hot, I’m always careful of this as I have once been burnt! Could it be an allergic reaction to the purple shampoo I used on her hair shortly after doing her brows??