Is my chicken spoiled?

I get the frozen tyson chicken at wal-mart, in the big bags. I usually thaw over night and cook the next day. This time I let it thaw/sit in fridge for 2 nights and when I cut it up this morning it smelled weird? Is it old? Did I let it sit in fridge too long? I thought it could be in there a few days before I cook it. I usually cook it when it is still partly frozen so maybe I am not used to this smell? Don't want to make myself sick! Thanks!