If this is your first time having grass-fed beef, it might taste different to you. I've never had "sour" meat, but maybe that's just how you're tasting it. Otherwise, yeah, I'd say it might have been a little old. Did it smell funny before you cooked it?
No, I've had grass fed steak a lot (except this was sirloin steak) and it smelled normal (the fat bits tasted buttery). Some parts tasted normal but the other parts tasted limey.
Oh, see, "sour" to me in meat is "rancid" in my head. If it stinks when you cook it, it's probably no good (unless you're pregnant, then weird things stink). If it smelled ok while cooking, then it's probably something else.