I have made cheese sauce before and it has never been sweet. No problem with butter and flour and then I added milk like before. White sauce was fine. So I added cheese. Yes it was a package of shredded sharp cheddar cheese from Harris Teeters. I know fresh is better but it was what I had. The sauce itself was fine, smooth and creamy, but it was sweet. I used skim milk and I have used packed cheese before but it has been a while. Is it the cheese? Have they added something or is it the skim milk?
Carbs in some foods (mostly those that contain simple sugars and highly refined grains, such as white flour and white rice) are easily broken down once broken down they are literally a sugar.
Cooking at too high a temp, or for too long, causes the carbs in the white flour to break down. I think that may be why your sauce is tasting sweet.