Breastfeeding to bottle advice

Tiff • Baby number 2 is on the way.
Hey ladies, any advice appreciated. I've made the decision to switch from breast to formula today. Baby is so uncontent on my breast and cries and cries when triying to suckle. The only time she seems okay is during the night and I think that's because she's so sleepy and out of it that she just takes it. 
She has just had bronchitis and has been I knew hospital and ever since then just doesn't seem the the same with breastfeeding. In the hospital she was fed my milk through a tube and it seemed to really satisfy her as she always had a full belly so I'm worried that she isn't getting as much of an intake now, hence why she is crying all the time. I must add that my baby is quite a big baby and was born at 10lbs, she's now 4 weeks old and I don't think she is getting enough from me 😥
Am I able to introduce formula straight away?