My Sam is now 5 months and I have been exclusively breastfeeding. She is now down to about 5 minutes on each side and her weight gain has been fine. Babies will get more efficient at breastfeeding and can get what they need faster. How far apart are your feedings? Maybe you could spread them out more since her tummy is getting bigger and can hold more. At 3 months Sam was about every 3 hours. Also Sam got fussy as she got older and now she can be quite fussy and distracted at daytime feedings while nighttime feedings tend to be more mellow and longer (she falls asleep).

That said I totally understand your concern over her lack of weight gain. Your doctor isn't concerned so that should be some comfort. It's hard to tell a new first time mom not to worry. (I am a new first time mom and worry all the time.) Sam has gained more weight than Hannah but every baby is different!! I just want you to know that some of the behavior you are seeing is normal for her age. How is her disposition overall - does she have happy playful times? Didn't I see video of her kicking up water in her bath tub? She looked quite fine there so I think you would notice something in her overall behavior if feeding/weight gain was a problem.

Another thing - I've read that the monthly friend can affect taste of the milk. Not sure if that is an issue yet with you but she may eat less then and it is temporary.