Symptoms and signs of being presented around a gas leak are-
light headed, head ache, stuffed and runny nose, heavier and more difficult breathing rhyme, forgetful and real air head like, tired, feeling off in general and you will also smell an off smell.
ways to prevent in future next time is to buy a smoke detector alarm that also specifies in also detecting carbon monoxide. In the meantime id suggest going around to all the gas pipes and fittings located internally and external; spraying with a bottle containing lots of soap suds in the water. you will know straight away where the gas is escaping from because it will continue to bubble up.