I had my first panic attack today which was day 2 of my contained water dives. Yesterday no problem. Today arrived late and got the last regulator on the rack which seemed a little hard to draw air from. Just didn’t feel right. First skill was taking off and replacing weight belt which required a small amount of exertion. Was immediately starved for air. My instructor was right there and checked to make sure my tank was fully on then handed me a slate (which kept me from shooting up 6 feet to God Blessed Air). I wrote "not enough air, high resistance to flow" having some physics background. He gave me my Octo which never occurred to me and lo and behold I had all the air I could possibly want. Finished 50 minute skills dive no prob. Went back to original secondary when tank pressure dropped to about 2000 and everything was fine. I will never forget the panic feeling though. The less experience you have the more important it is to be with a highly skilled buddy.