My daughter woke up with this huge spot on her foot I don’t know what it could be. She’s been in pain and can’t move her foot what is this.. Explain this

A dark, blood-filled blister suddenly appears on your skin. It looks angry, painful, and wrong. You tell yourself it’s “just a blister”… but what if it isn’t? What if it keeps coming back, or shows up in your mouth for no reason at all?