Inner ankle/heel pain

I woke up with a tender red area on the inside of my left ankle/just above my heel one day. It almost feels like a bruise, but there is no bruising. There was no injury that I can recall. It hurts more to put pressure on that foot and there is a fairly firm bump under the skin. The skin is red but not warm. It feels almost like a strain but is nowhere near the ankle joint. Any ideas?