Hi J.,
The back arching is a classic sign of reflux, so I'd definitely take her to your regular pediatrician first. If it is reflux, there are a few non-medical things that could help make her more comfortable, though they alone wouldn't get rid of the reflux problem:
1. If you are nursing her and she nurses lying down, try having her always lie on *her* left side. That would mean a cross-hold when she's nursing on your right side and a football hold when she's nursing on your left side--so you just slide her from one side to the other rather than flipping her when you switch sides. When she's lying on her left side, gravity helps the stomach juices head down into the stomach rather than back up the esophagus. If you're bottle-feeding, having her a little more upright while she's drinking could help for the same reason.
2. Keep her upright or semi-upright for a while (20 min or so) after she nurses or has a bottle. Again, this puts gravity to work for you, this time while she's digesting.
These two ideas wouldn't substitute for medical care, but they can help reduce some of the discomfort if reflux is indeed the culprit. BTW, though many babies with reflux spit up, a baby can have reflux even if she isn't spitting up. Your pediatrician can help interpret the signs you've been noticing in your babe.
I hope you find some relief for your little one soon. If it is reflux, don't worry--most babies have some degree of reflux because the sphincter that keeps stuff from coming back up into their esophagus isn't fully developed yet. As they get older, the sphincter strengthens, and they typically outgrow the reflux. Our little guy had it pretty bad, but he outgrew it by 6 months.
Take care! Juliet