No! Don't stop! I'm a prenatal exercise specialist and I struggle with the same thing. Last pregnancy I worked out about 5 days a week and literally one day to the next felt completely different.
Depending on what your body is going through at the moment, what you ate for breakfast, how much sleep you got, how warm the place is, how much water you drank, you will have more or less energy.
I know it's hard, but be easy on yourself, and if your body just wants to walk a mile, just walk a mile. Walking a mile is better than doing nothing. On days when you feel up to running, run. If you need to run-walk, do that. (It's kind of like how some days you feel like eating more and some days you feel like eating less. You don't force yourself to eat more on days you aren't that hungry, you just eat less that day.
Try to eat a complex carb + protein (whole grain crackers w/peanut butter, yogurt and fruit) within the hour b4 you workout and make sure you drink plenty of water. Sometimes dehydration can make a workout harder.
Good luck, and hang in there!