For a start you will need to double check that you didn't just see a regular old shadow, check if there are any objects, or collections of objects that could give you the impression of what you saw.
Next you need to rule out that it wasn't just your own mind giving you the heebee jeebies (and your behaviour giving it to others), as it is in many cases. When you get scared your brain often overthinks things, giving you emotions of dread, and feelings that something bad will happen, this is simply your brain preparing you in case something bad would happen because of what you saw, it does not necessarily mean that something bad will happen.
If you have gone a few days you forget about it and don't feel anything else (don't try to feel for something or you will feel it because you are thinking about it), it is probably safe to say it was your imagination.
But if you feel very uncomfortable, cleanse the area.