Is it strenuous exercise or sun exposure?

I had radiation only after a lumpectomy/sentinal node removal in Oct 2010.  Last summer, I would swim in my pool during the morning hours and noticed that afterwards (the next day), I would experience the same pain and lethargy that I had towards the end of my rad treatments just 6 months earlier, making me think it was sun exposure.  I also attended yoga classes on 3 occassions last fall and each time had a return of the same fatigue the next day, but no pain, just fatigue.  Other than that, my normal exercise routine consisted of riding the bicycle and the doing the stair climber for about 30 min max at my local gym, which caused no fatigue or pain.  Just a few weeks ago, I started walking 2 miles on my lunch break, which put me out in direct sunlight (I live in the sunny south US, so it is hard to avoid it).  This causes me to have pain in the radiation site and lethargy the next day.  Now I'm wondering if these symptoms are caused by sun exposure or swinging my arms while walking (or moving my arms while swimming).  I do use sunscreen and make sure the site is covered.  Anyone having similar experience?