Yesterday I took a 20 minute walk to the park to spend some time in the sun. After about 10 minutes of walking I suddenly felt markedly more exhausted. It wasn't too bad, just uncomfortable, and so I continued to the park. The exhaustion was still there upon returning home. As I woke up today, I immediately knew that I wasn't going get anything done. My back hurts, I can't concentrate, I'm irritable, and my energy levels are low. There is no apparent reason for the sudden worsening.