1989 300E

Very strange thing just happened to me. Filled up the car yesterday and drove it 10 miles home. Did not overfill, simply stopped when the pump stopped. Went out to the car and opened the gas cap today, rather than the normal hiss, gas simply gushed out at me. Not once but twice. What could cause this? Is there some one way valve that has failed in the vent lines? Never seen or heard of this before. No smell of gas in car or trunk either.

Any ideas appreciated.