Could it be possible that OP is speeding up your metabolism? I think the same thing happened to me. I've managed to gain some weight by taking the mantra "eat oil" seriously. During or after every meal, I swallow 1 tablespoon each of olive oil, avocado oil and coconut oil. Sometimes I just stir it into my food. Today I stirred the coconut oil into my coconut custard pudding. I use a lot of olive oil on my salad, probably three times more than I used to use before. This isn't hard to do if you have good tasting oil. It has to be doing good things to my insides and I am gaining weight, slowly but surely. A bonus is that any tendancy toward constipation is a thing of the past since I started ingesting oil, and I seem to have endless energy.
This whole concept of a low-fat diet is a bunch of malarky. It's been shoved down our throats for too long. It isn't a matter of low fat, but good fat. And lots of it. Even organic butter is wonderful for you. I'm starting to read that the whole anti-cholesterol campaign may be missing the mark. Our bodies need good cholesterol, like that found in organic eggs and butter. Cholesterol is necessary for body functions. When I think how much is spent on statin drugs and for what purpose, it makes me question our health authorities.