It's this feeling. The reason I was falling apart was because the ecto in my body couldn't hold together. I couldn't regain the energy I had lost, at least not at a rate I needed to survive. After a certain point, I had to forcefully will it to stay inside my body. I could feel it leaving and everything would start falling apart and I had to drag it back in before a limb turn to goo.
That flow of energy, when you take energy, feels like that.
And it would explain why it hurts in ghost form. There's a ton of ghost repellants that are repellants because they drain a ghost of their energy. Their ecto. And that hurts. And why it doesn't as a human, because then I'm not as sensitive to that feeling.