Run this morning

I just got back from a 40 minute work-out that was a run down to the river and back. On my run on Sunday, I overused my right leg, causing pain to shoot up and down my right side. For the first twenty minutes of my run today, my right side felt fine, as I was coming back, my right side started hurting so I switched to a run/walk pace until I could get back to my cabin. I don't know what is causing this pain but I am encouraged that the pain took longer to occur and now after taking a shower and eating, I don't feel much pain at all. I'll continue to monitor how I feel but I want to continue to run and exercise. I am planning my next run for Thursday.