Please help me. My dog is hurt but I don't know what to do.?

Today I let my dog out side to tinkle and I turned my back on her for one second. The next thing I know she yelped and started limping up the stairs to the back door. I got her and tried to figure out what happened. I checked and her bones aren't broken. I think she may have gotten stung by something but when I examined her leg I found no marks. She is limping pretty bad and will walk only if she has to but mostly she is staying laid down and the hurt leg keeps twitching. She is a 3 year old shih tzu. What should I do?