Mary, if you have a concern at all, since everyone's life could depend on it, call your gas company and/or your fire department. They have sensitive gas detection equipment that can find and detect different gasses. There could even be a leak somewhere else and it follows buried pipes into houses. (I remember some houses in Kansas City, I think, that blew up from such line leakage and there was a big deal about whether the gas company or homeowner was responsible for the lines and I think the gas company replaced a bunch). By the way as a warning, even if you call the fire department non emergency line, some of their policies still says lights and sirens on even non-emergency responses.
Take the safe route and have it checked. They don't charge for it. Especially now when everyone is couped up because of the cold weather. Slight headaches is a symptom of light carbon monoxide poisoning so ask everyone.(Occasionly someone in the house will survive or be found in time and they remember the headaches they had leading up the the fatal event) A storm could blow in and cause your furnace vent to downdraft and a minor problem before could be a deadly problem. I wouldn't necessarily trust every furnace repairman to catch a cracked combustion chamber or a vent drafting problem if it isn't doing it when they are there. If they find some type of leak, they can put in fans, etc. and be ready to get moved to a motel.