As with the comments above, the creative process is all about continually improving upon your original concept. If your mind is working and you've come up with new ideas, that's great! Many people struggle to get even one idea down, so if you're thinking up lots of ideas, that's a good thing!
If you really want a short story, I suppose you need to look at what the core concept of the story is. What message are you trying to deliver, and do these extra ideas/plotlines help or hinder that main idea? Is the short story really gaining anything by having these ideas?
If the answer is yes, well that's still a good thing! It just means you've probably moved into novella territory, which isn't a bad thing at all. If you think, though, that you still want a short story (for publishing in a magazine for instance), then I'd recommend you have a look at which ideas/plotlines are really true to the core storyline and essence of the story.