I'm betting it's a bug bite of some sort. Or a rub mark from the new shoes.
Put some A&D ointment on it at bedtime, if it's gone in the morning, I'm betting it's the sandals. If it's not I'm going with bug bite, for which I'd put some cortisone cream on it, and see what happens. Redness means irritation. There can be a million and one causes. At least she isn't itchy or in pain.