I made a bit of a mistake, and in a bit of an accident I passed onto Karlach the information you had allowed Astarion to tell me, as well. I did not want you to blame Astarion for breaking your trust - it was my mistake through and through, and I felt very badly that you might consider him less trustworthy for something that was entirely my fault.

I, of course, will tell no one else and apologize profusely. It was stupid of me.