How can I get rid of pain in my tendon?

The (Achilles?) tendon in my left leg hurts when I walk a lot or swim. It also hurts after I sit with my legs crossed and in certain positions when I sit with legs touching the ground (I don't know exactly which positions). I think it started when I tried to consistently swim faster than I used to. I returned to normal pace, but the pain remains. When I don't walk a lot during the day, I feel no pain.