kitty,
((HUGS))......Long time, no see. Anyway, I just want to say that there may very well be nothing 'wrong' here, but I am sure you know that. One thing that creeps up as a possibility is teething. Yeterday, my 8 mo got her first tooth. She has been taking only 4 oz in each bottle for the past 3 days (she usually has 7 oz). So, I am assuming that is what it is with her. Maybe your LO is also teething? Also, I am sure you have realized that she might be picking up on the tension that feeding time brings out in you. I know that merely telling you to relax sounds a bit patronizing, and I am not trying to come across that way, but it is possible the activity of feeding is causing anxiety in you that is rubbing off on her. Maybe you can take a few steps back and have someone else feed her?
DD1 has always been tiny. At 8 mo, she was also drinking only about 6 oz, ate about 3-4 oz of solids and weighed only around 15 lbs. She turned out ok (but is still quite thin), so I don't worry quite as much about DD2. Maybe you had the opposite experience with your first, and now this is where your worry is setting in because it is new to you, kwim?