The backing away from feeding, arching back and a lot of crying- also sudden crying when she was asleep (as if someone hit her and the suddenly back asleep like nothing happened) were sings of silent reflux in my daughter which was not diagnosed until she was 3 1/2 months. She was prescribed an anti-acid and it worked. I slept my first full night after she took it the first day! She was also very gassy so they gave us anti-gas at first, but it didn't work because it seemed that it wasn't her tummy but her intestines that had trapped gas, so the anti-gas didn't do much. If she's also spitting up a lot then I would say she's got the classic symptoms of acid reflux (as described to me). When I fed my daughter and she got fussy I'd put her on her tummy and she would stop almost right away... being on the tummy really helped.
The runny poo- mustardy also sounds like my little girl. I gave her fomula and breastfed her as well. I worried about the consistency, etc.. since she was born premature and her intestines had not developed well when she was born, but at around 6 months and when she started eating solids, she just started to get firmer and firmer stools- and a more normal colour (green, bright yellow, and looking runny with rabbit droppings things...in different colours!).