Well it may be that you are switching to more "non-natural" things.
I have no idea what exactly you switched to but for example:
Regular maple syrup is loaded with calories but it made with sugar, which is more "natural" than sugar-free sugar that has very little calories but sweetened with chemicals similar to Splenda. I thought of this sugar example when you said "low calorie jams" which may be cutting out the sugar to lower the calories, but adding in chemicals to keep the sweetness.
Personally, when I eat foods with artificial sweeteners or artificial ingredients, my stomach does not like them!
So take a look at the food labels of the foods that are new in your diet. Look for words you can't read
or anything that it not "natural." That would be my best guess because that is what bothers me.
Remember that the more whole and fresh foods you can eat, the better! I definitely think it is more important to eat good things under your calorie limit, rather than eating a bunch of processed "low calorie" foods under your calorie limit.