The dog is obviously in pain. I would recommend having a vet check her out. It's impossible to get a diagnosis online. (Even if a vet was reading this, he/she couldn't give a diagnosis. Examining an animal is a hands-on process.)
shes in pain. she may have a broken bone(rib) or maybe some bruising. check under her fur to see if you can see anything. when did you get her? was she being abused? check it out with the vet.
first off, is it a cry of pain or a moan or a growl? If its a growl it doesn't like you
if its from pain...look for anything from bumps to red spots to ticks or fleas...if theres nothing obvious I wouldn't know what is wrong with your dog.
It might just be uncomfortable with you touching his side?
If you feel like something is wrong, feel free to get it looked at it.