Constant ache

[Usav]

I walk everyday for the last 5 years . Long brisk walk w uphill steep slopes 8km in hour. I walk no tiredness no troubles at all..but during the day i feel Stable aches on my left arm sholder hand together..sometimes tip of my left side of stomach just under chest rib. I lay down face up on sleep no problem . At walk i dont feel it. When i sit at home it starts . I cant stop it .what could it be .any advice