You should go see your primary care doctor
about this especially if the spot is spreading. They can take a look at the red spot and figure out whether this is something that needs treatment or not.
I say this because, occasionally, a red spot like this can be a sign of an evolving skin infection, called cellulitis. Signs of cellultis include reddened areas of skin that spread with accompanying warmth, pain, and potentially fever
. Cellulitis generally needs to be treated with antibiotics that your doctor can prescribe for you if they are necessary.
After this exam, and after ruling out a skin infection, then the most likely possibility will be that the red spot under your glasses is just a sore spot from pressure from glasses that fit too tightly. In many ways this would be analogous to having a blister or sore red spot on your foot from a shoe that fit too tightly. If this turns out just to be a tight spot, then you will most likely need to go back to the eyeglasses store or optometrist
who fitted your for the glasses and ask them to resize the glasses for you.