So that’s what a losing streak feels like. Stocks fell for the second day in a row Friday, which hadn’t happened in a month, as Amazon put a scare into yet another industry: medical device and health care equipment companies. Those companies slumped after an analyst for Citi Investment Research said Amazon might be on the verge of shaking up their industry by speeding up distribution and cutting prices. Energy companies gave up some of their recent gains while retailers, media companies, and household goods companies moved higher. One factor in those losses was uncertainty over the Republican plan to cut taxes. ‘‘We would expect a little bit more of that as we get more delays and uncertainty in the tax plan,’’ said Sean Lynch,at Wells Fargo Investment Institute.