What's wrong with my Achilles?

A week ago I went on a hike and I don't know what happened because I didn't start feeling pain in my Achilles tendon until the day after. It still hurts to put too much pressure on that foot (especially going up and down stairs and exercising), and I just noticed tonight that there is a lump there. There's no discoloration like a bruise though. Is this something I should just wait out or does it sound like it can be potentially serious?