Gaining

I gained about 15 pounds (135 to 150) drinking heavy cream for a couple of months. I stopped drinking it, didn't change anything else, but now I'm back to my original weight.

Has anyone had a similar experience? I don't want to have to keep drinking heavy cream to maintain my weight.

How much cream were you drinking?
To me it sounds like you're not eating enough calories without the cream to maintain 150, so if you try again, I'd suggest counting calories to make sure you're getting at least enough to maintain.

Your body likes being at a certain weight, and this is called the set point. From what I have read, the set point can increase with weight gain, but it never decreases.

I think what's happening is that you haven't gained enough weight to increase your body's set point. When you stopped drinking the heavy whipping cream, your body just returned to its set point of 135 pounds.

I have had this happen in my weight gain journey. Just last month, I resumed my normal diet in between gaining cycles and lost 10 pounds, dropping down to 260 pounds. Once I reached my set point of 260 pounds, the weight loss stopped.

My regular diet consists of eating the same foods and amount of foods that I ate prior to gaining, back when I weighed 166 pounds. Due to raising my set point, I can now eat this regular diet, and my body won't drop below 260 pounds.

I think the solution to your problem is to diet for about a week, and then try gaining a bit more weight. Instead of 15 pounds, perhaps gain 20 or 25 pounds. The combination of dieting followed by a larger amount of weight gain should help raise your body's set point. Once that happens, you should be able to maintain most of the weight gain without having to drink heavy cream.