my 13 week old has gone off her food!! is this common?

My little girl is 13 weeks and used to take 30oz daily. Since having her first injections its a struggle to get 15oz into her daily. She still seems happy and content. She is a big baby and weighs 15lbs 13oz already. I just dont understand it? She slept 6-9 hours each night from 10 weeks but now she is back to waking every 4 hours. Ive had numerous discussions with her doctor and health visitor this week and they dont seem concerned. I hate her not eating! Have any other mums experienced this? Thanks, Sara