Feeding pattern of preemie has gone to pot after immunisations.

[Becr811]

My daughter was born at 31 + 3 and is now 9 weeks old and in the past 4 days since her immunisations has completely lost a feeding pattern. She has been home 5 weeks and is putting on the 1oz a day expected.
Before she was virtually 4hrly on the dot (SCBU put her on this) and was eating about 100 to 120mls of 50/50 EBM and nutriprem 2 as we are having to move her to formula. She was always a hungry baby even though she has silent reflux which is being managed by renitidin and occasional gaviscon,
Suddenly she is very sleepy and we are having to wake her for feeds which are taking 3 times as long as before and she is refusing half often. She has also started being sick a lot more (about 10 to 15ml) after some feeds. The Dr has said to force waking her up and feeding but if she doesn't want it we have no chance. She doesn't appear to have any other symptoms and was looked at by a Dr who agreed it might just be her immunisations throwing her off.
Basically has anyone else had a preemie who suddenly wanted to eat less often but seemed in every other sense to be happy. This is really stressing us out as we don't seem to be able to do right with her food past few days. Should we try feeding her less to get her to wake up on her own?
I will talk to my HV on Tue.