Does stress impair your short term memory?
It really depends on the timing of the stressful event, as well as anticipation. From an evolutionary standpoint, "stress" developed for a reason in the context of heightened arousal, attention, preparation for action, etc. Say you encounter a lion in your backyard: Not only will your sympathetic nervous system be on full speed, but several cortices are also geared up to processing what's going on. The idea is that you want to make a quick behavioral response, so inducing stress short term actually facilitates many different processes, including encoding of memory. See the link below for some research on this, I grabbed one that should be open access.

http://www.ncbi.nlm.nih.gov/pubmed/22483973

Basically short term, acute stress may help encoding of memory, whereas long term, chronic stress not so much. Also consider whether or not you are learning something and then hear a loud human scream in your ear. This may disrupt encoding. So again, it's the timing that is important. And I would say it depends on the type of stress.