You're just in full flow and then someone else stops by and interrupts you. Now you're back to square one.
Know the feeling?
Almost everyone does, but for some people this is the pattern of their whole working life. Learning to manage the flow of visitors so that you can concentrate on your work can leave you a lot more productive.
Techniques include advertising a time when you're free to talk in a communal Outlook calendar and emphasising those when you are not free too.
Action step: Why not take 5 minutes to talk to your IT department about communicating your Outlook calendar now?