2002 Nissan Sentra white smoke

Oil light had been intermittently coming on and flickering periodically over the past month, had it checked, oil was fine. Change oil every 3,000 miles or less. When I would add 1/2 qt of oil, oil light went off. Recently, tried to make a round trip of approximately 250 miles. On return, stopped for gas, oil light had been flickering, added 1/2 qt oil. Checked oil afterwards, it was a little over full. Turned car on, started to leave, service engine soon light came on and a lot of white smoke billowed out of tail pipe, smell of antifreeze on outside of car. I stopped, checked antifreeze and radiator to make sure no busted hoses. Turned car back on same thing. Temperature gauge did not register hot though hoses to radiator were very warm. No sign of overflow of antifreeze on engine, and plastic overflow jug was full. Went to rear of car while running and felt in tail pipe and water in tail pipe. Sounds like a head gasket to me. Drove approximately 1/2 block, white smoke continued to billow out the tail pipe. Drove back to gas station and parked it. Sounds like a head gasket. With the aluminum block - if it did not overheat, what are the chances the head is warped and could it be something else?