Back of foot swollen and tendon tender, why?

So I have been on my feet a lot today, and I wore flip flops to work. Well when I got home I noticed that the back of my left foot hurt, like the achilles tendon area when I pointed my foot up or bent my knees. My right foot is fine. I then looked at my ankle and noticed on the outside of my foot between my tendon and my ankle bone it is swollen and puffy, not too bad but enough that you can tell. My tendon also still hurts and hurts when I rest it on the recliner or on my other leg. I do not remember bumping anything. I was on my feet a lot today at work because we were busy and I work in sales. I currently have an ice pack on it and will take an epsom salt bath a little later. Could this just be cause by being on my feet in flip flops? Or did I do something to my foot?