Thank you both for your feedback.
My body fat hasn't changed measurably in one direction or the other, so the gain might be a little muscle.
And I tried increasing my calories by about 500 and I gained quite a bit in a short amount of time.
I will say it again: I'm a freak. You can't even imagine how frustrating this is.
Nah, you sound like me. A few questions:
1) Are you at least losing inches?
2) How much water are you drinking? How much salt?
3) How balanced is your diet?
Whenever I hear this, my guess is it's one or more of the following:
1) Too little variation in exercise. Your body adapts quickly to what you're putting it through. Provide more variety in what you're doing in order to exercise, and don't just do a lot of any one thing.
2) Water retention. Drink at least 10 cups a day and watch your sodium intake.
3) Not burning as much calories as you think you are. Everyone burns differently depending on metabolic rate, age, body type, gender, et cetera.
4) Eating more calories than you think you are.
Were it me, I'd vary up the exercise and try reducing the calorie intake a bit to see what happens. The way to know if you're not getting enough is if you get too tired to exercise. THAT'S a warning sign. You need fuel to burn fuel, kinda like spending money in order to make it.
Hope this helps, even if a little bit....