Last night our 8 year old lab woke us at 2 am screaming, it sounded like he was seriously hurt or dying. My husband and I ran to the room he was sleeping in to find him sitting in a corner, still screaming, and shaking like he was terrified. His heart was beating so hard, it was like he was having a nightmare. This is the second time this has happened this week. Last time it was during the day and he limped a bit after, we checked for injuries both times and found nothing. After he calms down he stopped limping and even ran around a bit playing with our other dog. Please help