Broken, damaged, incomplete connection on the coax is most likely your issue.
Any half decent wire/connection along with antenna that far away, there should be zero affect on your reception when you move about inside your home.
To me, it really sounds like your outdoor antenna is not connected for all intents and purposes and the coax inside your home is actually acting as the antenna..
Perhaps state exactly how the wiring is run, is there any barrel connectors, switches, splitters, wall outlets between the TV and antenna?
Additionally list the make and model of the antenna preamp and the antenna while you are at it..
With proper connection terminations and good quality RG6 20 ft of coax is nothing.. I have nearly 50ft of coax from my antenna going into my basement and loops into the living room at the far end of the house to the power injector for the outdoor preamp, then runs back into the basement into a amplified 8 way coupler (not a passive splitter)..
Body movements inside the house do not bother any of my channels..