warmup exercises = doing a couple sets with 50% normal working weight, 12-14 reps.
static stretching a cold muscle is more detrimental than helpful...you can pull muscles this way.
so, its either dynamic stretching ( you can buy a video online that explains it ) or just do warmup sets for compound exercises. no need for isolation warmup because those SHOULD be coming at the end of a workout, or at least after the compound move, that warmed up the large and small muscle groups at the same time.
and as far as shoulders go...what exercises did you do yesterday? there are some common exercises that are really not good for shoulders/rotator cuffs.