Was taking an examination or had been taking?

[Frieda]

I missed the meeting yesterday. But I was (or had been?) taking an examination?

[Josef Essberger]

It's hard to say because the paragraph lacks logic. Do you mean because, not but? If so:

I missed the meeting yesterday because I was taking an examination.

[Frieda]

Yes I mean I could not be at the meeting, because I had something else to do.

[Josef Essberger]

I missed the meeting yesterday because I was taking an examination.