I was in the middle of doing an activity. The doing of something was in progress in the indefinite past. It must have started but it is not known if it ended.
However, I don't want to add any other activity or a specific time such as on Sunday, yesterday, or at noon
'I was doing' without further context seems wrong/incomplete because usually we say 'I was doing something on Sunday' or 'I was doing something when you called me'.