Something is moving under my skin.?

This is no loose muscle or loose cartilage or something. There is literally something moving by itself inside my left arm. Every once in a while, a 2-3cm lump is moving under my skin and disappears after about 4 seconds. My sister doesn't believe me, my parents don't either. I recently went on a tour in the tropical areas in Northern Vietnam Lowland Rainforests. I'm really worried so should I see a doctor?