difference

I was working-- there was a specific time/event "I was working yesterday at my job"
I have been working-- it is a frequent activity "I have been working a lot this month"
[does this make sense?]

I was working - at a specific time (I was working when the boss arrived) or during a period (I was working all Saturday) in the past. The speaker is no longer working.

I have been working - at specific times or during time periods in a time period continuing up to the present. The speaker may still be working at the moment of speaking (I have