Nicely done! Really enjoyed that. It could clearly be part of a much bigger story and it made me interested in seeing more. That is EXACTLY how a short should be. So many people make long shorts or even feature length films that just drag. No, your concept wasn't the most groundbreaking in the world - but everyone knows that dream or feeling of being scared - and I think you hit that very well. Camera shots gave so much to that, would have been a completely different feel otherwise.
I can see everything done was done as best it could given a small budget, and no attempts to push the boundary of what you could do well. Again, in reference to others - so many will show too much of their effects and all it really shows is how fake looking their effects are because they just can't pull it off. You show what you need to, and it looks GOOD because you know your limitations, and you were able to film it in a way that doesn't look like you were just trying to show off.
Acting works, wise use of limited dialogue. Props - great, not everyone has a weapon, these are real people and it makes it more believable. Even the contacts at the end looked good but you could have easily overdone the visuals in that moment. My only thoughts for any changes would be the synth soundtrack and maybe the visual effect at the end where everything is polarized/negative. But, just my opinion ;)
Lovely work, will be looking forward to more and I hope you progress! Have you thought to try kickstarter or indiegogo for funds? It would be wonderful to see what you could do with a bigger budget!