Fussy eater - starting expressed milk

My daughter is 12 weeks old now and was feeding fine on the breast for the first 10 weeks and sleeping from 10:30/11:00pm through to 4:30/5:00 am.  Then suddenly she has startd to get really upset on the breast and is now only sleeping till 2:30am then waking again at 4 and 6.  She feeds quite happily for the first 10 mins then comes off and burps.  I offer her back on and she feeds for about 2/3 mins then starts kicking and screaming, but trying to latch on at the same time.  I try winding her and that doesn't seem to be the problem.  I have tried swapping breasts and still  she yells.  It means that we both end up getting upset.  However if i take her off then she is quite happy, not routing or anything, but if i offer she goe to latch on and then yells and kicks.  

Since she has been doing this she is waking more at night and needing to feed, therefore i am concerned she is not getting enough during the day to see her through to the morning.  I have started giving her expressed feeds for three of her feeds in the day as she will happily guzzle down 7/8 fl oz,  I only started yesterday evening so don't know if it will help her to sleep through again.

Does anyone have any ideas why she is being so upset on the breast and if I should just give her expressed milk?

Also i can normally only express about 5 fl oz a time, but she gets upset when its finished so have been offering her another 2 or 3 fl oz, which she has drank happily and probably would have drank more if i had it!  How much should she be having?  She is 12 weeks old and weighs 11lbs.  Is gaining weight well on the chart, still on the same percentile as she was when she was born at 7lbs 3oz.

sorry if its a bit rambling!