If you don't know how to do it yourself, get any propane or natural gas burning appliances (water heater, furnace, oven, etc) checked for proper venting. If you're burning wood, or other solid consumables, double check your flue damper position (sometimes people will shut it down so far that it won't allow sufficient amounts of exhaust gases to escape, which can build up CO2 & CO levels).
Also a good idea to break out a ladder, if you have one, to check your exhaust vent roof penetrations. Lots of times a bird will nest in one during the warm weather months, and block it from venting properly come winter, when you're wanting to run your furnace. While you're up there, go ahead and make sure your plumbing vents are clear too.
I know it can be expensive to bring in a professional, but it's worth the money when you consider that ignoring the problem could cause you, and your family, your lives.
Hope you get things fixed quick!