Help!

Thankyou to anyone that has took the time out to read this, I need help! My lo is 17 weeks old, she was born 9 weeks early at 3lb, she's now over 10lb and has had milk since day dot. About 2 weeks ago I had the feeling she was starting to want more, but as the hv disagree with weening before 6 months I decided against it (although both my older daughters where weened already at this age) well anyway now for the past week she'll take 1-2oz and will turn her nose up at it, we've tried faster teat and different temps for her milk, it seems like it's too much hard work for her when she's not getting enough. But now she's like this it seems she doesn't like the taste, she sleeps all night and will be awake all afternoon and evening moaning, this is when she takes the most milk. I can't change her milk as she's on a prem baby milk untill 6 months and I'm scared her stomach won't tolerate weening. Has anyone else been through anything like this? Any help or advise would be amazing and thanks again for taking the time out to read