This pain may be because of acute muscle spasm in lower back muscles may be associated with heavy work or sitting down.. The strain over muscles increase on sitting down while washing clothes thus increasing the pain..
Next Steps
I will suggest you to start hot fomentation, avoid sitting down and working, take analgesic with muscle relaxant tablet, analgesic ointment over the site of pain 3-4 times daily..
Health Tips
Avoid heavy work.. Plan for x ray if not relieved..