If you were having this problem while after diving, not snorkelling, it would probably be mask squeeze - the air volume within the mask contracts at depth due to pressure differential between the air and surrounding water, pulling the mask hard against your face. The solution to that is to add more air to your mask through your nose whenever you feel the mask "squeezing" against your face.
But if you're on the surface, it's probably a poorly fitted mask, even though it may feel comfortable. Unless you have an older style mask, with a non-silicone skirt, which may be just too hard for your skin. I suggest you try holding the mask against your face, without the strap around your head, and see if you can inhale through your nose gently and let go with your fingers, and keep the mask on your face without holding on to it or using the straps. This is usually a good indication for a well-fitted mask. You should be able to create a seal around the mask with just your inhalation. If it doesn't stay on, it's not the right shape for your face, and you might want to try a new mask. Or you could switch to goggles for snorkelling in the pool (but never did with goggles! you can't add air to equalize them and you get eyeball squeeze instead).
Ben