What is her nappy output like? Has it decreased?
There are lots of reasons why a 4-week old might be crying or upset, it doesn't necessarily mean it's an issue with feeding - even if it's happening at the breast. She is happy to feed when sleepy? So maybe concentrate on offering when she's just woken up or feeding to sleep. If she doesn't want any more, try something else - cuddles, rocking, or wear her in a sling.

Expressing is not a good guide for your milk supply, if that is what concerns you. It only tells you how good you are at expressing (or how good your pump is). Also, even very well-fed babies will often apparently skull down a bottle of milk - simply because it flows so much faster and easier than the breast. The fact that she power-chucked afterwards may be an indication that she was actually over-full.

Seeing your doctor or an LC is a good idea. Always best to rule out any problems. She could be unwell even, as we head into flu season. If she's been gaining weight and growing well up to now, in all likelihood this is just a little blip and you'll be back on track again very soon.