Acid Reflux? Or something else?? HELP

i have an 8 week old daughter, for the first few weeks of her life she was a greater feeder and rapidly put on Weight. But now feedings are terrible. She has 1-3oz and will stop and start refising more and when I pick her up to burp her she throws herself backward and screams. the only place she doesnt cry is when we put her on our shoulder. Her weight has started to slow down because she just won’t eat enough. She has hiccups a lot also. 

I will add, she is NEVER sick and has  no problem sleeping at night and doesn’t mind being on her back. Which I know are reflux symptoms. But the feeds are the same every time. She is on HiPP Organic milk. 

I have been perscribe ranitidine, but so far no improvements! 

This feeding has caused me to develop mild PND and bad anxiety. Just don’t know what to do next 😪