Whu isn't she feeding?

my daughter is 4 months and 26 days old. However, since a few days she's been very fussy and it is so difficult to get her to feed properly, it's almost a wrestle. What she does is she takes a sip or 2 and then starts pushing upwards and pressurises her body to be very stiff and straight. Because she exerts such force while doing so,she goes red. I am getting worried as she's already weak and has also stopped wetting her diaper enough. Please help if you seem to understand what's happening here because I am seriously concerned.