Yes you must go to your Vet this is too serious.The amount of pain she is in sounds like broken ribs as well as other probs mentioned.
She has to have treatment as she would be in agony.
your best bet is to go to your vets office asap becuase it could be a number of things such as an old animal bite, a tumor, or an infected hair... either way it's best to go to your vet's office. Hope your dog gets better soon