There are a few possible explanations. The one I'm leaning most towards would be residual energy, and that is something everybody collects. Perhaps you are just more aware of it than others.
Residual energy could come from just about anything and it lasts a long time. One example of a person leaving and another collecting would be an angry person holding an object, we'll say a stove handle, while furious and just gripping it tightly. Then, they let go and wander off. An hour or so later, a person sensitive to residual energy wanders by and happens to, for whatever reason, grab that stove handle. He/she may suddenly feel that anger, but in most cases he/she will realize it's residual even if they don't realize that's what it's called.
Another possibility is that you are a psi-vamp and are feeding subconsciously, which is actually common, but since you've been practicing you have become more aware of energy and how to sense it and so have been becoming more aware of it. I have doubts about this, but since it is a possibility I figured I'd mention it.