| Obviously this is a little late (have not been online for several days), so hopefully your pup is back to normal by now! The condition you describe is called paraphimosis--the erect penis can become large enough so the skin of the sheath tightens around it and cuts off blood flow. Since the blood can't drain out of the area the penis stays enlarged. They sometimes go down on their own, but may have to be repositioned under anaesthesia.
Its sort of like the commercials that warn men "If you have an erection lasting more than 4 hours, seek medical attention..." The same thing applies here.....! | 
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