I'm Checking Info On Why I Would Develop A Fairly

I'm checking info on why I would develop a fairly large bruise on left arm bicep, for no known reason. Can't recall any injuries or strains. Bruise is nearly half size of the bicep, but I'm not really built like a body builder. Started 1 week ago soon after woke up. Very painful arm until Thursday, less by weekend.
By Sat. night a shadow of a bruise began to show. Bruise large and colored Sunday & today.