"slightly upset" is correct and usable in written English.
You can use it when you want to describe someone who is feeling slightly, but not too, distressed or emotionally disturbed. For example: "After hearing the news, Brad seemed slightly upset, but he was still able to crack a few jokes."
Yes, I am slightly upset by it, because I'm only 31.
But my constituents loved it," he claimed before conceding: "I'm slightly upset because I feel it was slightly undignified.
A proposed French law (well, an expansion of an existing law) has supporters of Android and other non-Windows operating systems slightly upset.
Every time I get slightly upset about things, I remind myself how lucky I am.
I thought the truth needed to be told, so that was much more important to me than people having slightly upset feelings".
A bomb threat at the Frankfurt exchange, which led to the evacuation of all floor traders for about an hour in the afternoon, only initially, and slightly, upset share prices, according to dealers.
On Nov. 16, 1934, during a provincial inspection tour by Emperor Hirohito, the imperial motorcade took a wrong turn, slightly upsetting the official schedule.
As a consequence, Claudie seems more vulnerable now, less in control, which both slightly upsets the balance of the play and gives it a new layer of feeling.
Patients rated the DA highly with respect to information clarity, usefulness and were positive about its use in practice, although 40% found the prognostic information slightly upsetting.
It's slightly easier to upset people with movies.
I knew what I was doing at the time, though, so I'm just slightly less upset than I normally would be when you sell ten million of something and don't get paid for it.
