At least they are asking and giving you some feedback! Most of the time, I got the standard "you are doing a good job" as the only feedback for my entire rotation----if that much! My whole team got unexpectedly burned on a surgery rotation last year with no warning that we were going to be ripped apart and it totally sucked because we had no way to defend ourselves.
This a hard question to answer. Here is what I did:
On my third year medicine clerkship, the attending pulled all of us into her office individually and asked us to tell her what we thought our performance. I think that I said that I felt as if I was working hard, knew my patients well, and usually could generate a fairly decent plan of care. I wanted to be truthful, but yet not sound pompous or arrogant. She agreed with that and added her on input as to what she saw as my more positive attributes.
Then, she asked me where I thought I could improve. That was a little harder because I didn't want to draw attention to anything that she may not have already picked up on. However, she put me at ease by saying that anything that I told her (or any negative feedback she had for me) would not be reflected in my evaluation if it improved by the time the rotation was over. I had missed a couple of acid-base pimp questions, so I told her that I needed to work on acid-base balance and a couple of other areas of weakness. She agreed with that, but told me that my overall knowledge base was very strong and she wasn't too worried about those areas of weakness that I had identified at that point. She then identified a couple of things that she would like to see improvement on and even told me specifically what she wanted me to do to improve. She was awesome! Definitely one of my better attendings!