If your pediatrician wasn't too worried about it, try not to worry yet. My nephew is 4 month old now. At his 3 month checkup he was already over 18 pounds. I think it depends on the child and how consistently they put the weight on. Your pediatrician would have access to that information and is tracking her growth. Unless she put most of the weight on in 1 month, I wouldn't worry yet. It sounds like it may just be normal for your daughter. All children are different, too, so you can't gauge normal growth for your daughter by how your other children grew. My daughter grew well and was always around the 80-90 percentiles. My son had a few problems at first but since the problems were worked out, he has steadily been growing at the 50th percentile.
It sounds like you have had enough to worry about with your little girl. This doesn't really sound like something to let yourself get upset about. If you really feel there is a problem, ask your doctor about it again. Your doctor can check her chart and tell you whether she has been steadily growing at a certain percentage or how much or a change there has been in your daughter's growth pattern. If you have her weight and height information for the past several months, you can check for growth charts on the internet or in different baby books and plot the chart yourself to see if she is on a regular growth pattern. Then if you have further questions or concerns, by all means talk to the doctor and tell them your concerns. If you are not satisfied with your doctor's answers, you can always get a second opinion.