6 month old on a diet??

My 6 month old daughter has not been feeding well since almost 2 months now. She nurses for 3-5 mins only in every nursing session. Her weight gain is very slow and sometimes it is extremely less. Makes me think sometimes if she is on a diet! The paediatrician tells me to express some and give her after she is done eating by herself. I have been doing that but it is not possible everytime. Is anyone else facing the same problem? And what do you think I should do to fix this so that my LO nurses properly. There is no issue with the milk supply. So I'm not sure what the reason could be.