Baby eating less?!

LJ • 1 baby girl with another baby on the way
Hey girls! 
So Isla is now 12 weeks and exclusively bottle fed. Normally she would sleep 7-7 and take 6oz of formula every 3hours. She's been doing this routine since 6 weeks old (obviously not 6oz weeks ago but same timings etc) 
So anyway - the last 3 days she is only taking maybe 3/4 oz in a feeding and I'm having to really push those on her - I let her go 4 hours but even then she's not interested in the food - I decided maybe she was distracted and started to try and do feedin in quiet rooms but still refusing her normal amounts. As a result of this is now wakening at 4am for 4oz then back to sleep till 7/7.30. Of course it's still a great stretch and it doesn't bother me at all to get up with her if that's what she needs but I'm just concerned as to why she's gone off her food =\=\ she's still very happy and alert and content and having wet and dirty nappies but I suppose I'm just worried. I might take her to get weighed tomorrow just to ensure this won't affect her growth but is anyone else experiencing the same?