My dog has a soft hip lump and is losing weight. What could it be?

Unknown lumps and bumps are virtually impossible to diagnose without, at the very least, a visual inspection of the area. As such, I will provide you with some general information. An unknown mass or lump is typically one of the following: a cystic (fluid filled) structure, an abscess (fluid-filled, but infected), a mass / tumor, an allergic reaction / insect bite, or the result of a traumatic injury. A thorough physical exam and inspection of the affected area would be needed in order to determine which is most likely. In many instances, a needle aspirate (small amount of fluid obtained from the lesion and placed on a microscope slide) or tissue biopsy would be required to obtain a definitive diagnosis as to the type of lesion. If this is not an option, one must consider the location, distribution, duration of onset, patient breed / age / sex, and a variety of other factors. As Foxy is a younger dog, it is less likely that she would have a malignant tumor on her body than if she was elderly. You mentioned that the lesion is soft - this is typically a good sign that she is dealing with something benign or harmless. Whether or not this mass has anything to do with the weight loss is very difficult to say. I would recommend you make a regular appointment to see your vet and have her assessed as a whole. Hope this makes sense.