An AmE slang expression for the circumstances OP describes might be a “downer.”
downer noun: slang 1. A depressant or sedative drug, such as a barbiturate or tranquilizer; 2. One that depresses, such as an experience or person. See, The Free Dictionary
The epitome of a “downer" personified, is “Debbie Downer”.
Debbie Downer is a name of a fictional Saturday Night Live character who debuted in 2004, and who was portrayed by Rachel Dratch.
The character's name, Debbie Downer, is a slang phrase which refers to someone who frequently adds bad news and negative feelings to a gathering, thus bringing down the mood of everyone around them. Dratch's character would usually appear at social gatherings and interrupt the conversation to voice negative opinions and pronouncements. See, Wikipedia
Debbie Downer at Yahoo!