I have been at the beach for 4 days. been doing a lot of walking. noticed slight bruising behind both knees. should i be concerned. no pain?

Bruising.: Your question is greatly appreciated. It is often difficult to provide a comprehensive answer without more details. Some of the more worrisome symptoms would include easy bruising, easy bleeding, skin rash, fever, fatigue, weakness or numbness in the extremities among others. If the condition persists or causes you concern it is best to be evaluated by a doctor.