So there is no definitive way I will be able to tell whether your beer is having an issue or not from what you have stated above. However, it doesn't sound like one of the biggies:
Butter - Sweet Corn - Metallic (penny) - Rotten Eggs - Bananas - Skunk - Cardboard - Musty / mildewy - Cloves - Green Apples - Vomit - Vinegar - Cheese - Plastic Band-aid
So the fact that 'cheap lager that has been left out all night' is at least not close to one of these should at least give you some hope. It may be a combination of two or more of those above but I'll remain optimistic and assume that the smell you are getting is one of the many strange (but normal) smells given off by our little friends. There are many online resources to help with off flavors and smells online. My search brought up this one which seemed to explain each of the smells I knew of and more:
http://www.belgiansmaak.com/off-flavours-in-beer-off-flavours/
Good luck and cheers!