What Causes Circular Indentation Below Both Of My Knees In A Hot Bath?

I have noticed a ciruclar indentation below both of my knees, they only appear when I a in a hot bath. They are not red/irritated. I first noticed them when I was on high dose prednisolone for neuromyelitis optica last year, I had oedema in my legs. Is this anything to worry about/ I have been off treatment since Oct 10