First, take care of your dog. Get to a vet, and if you cannot afford it, ask them about payment options or care-type credit services. Depending on where you live, there may be a teaching hospital available. Do not let your dog suffer, it could have a broken or severely sprained leg, and if kicked, internal injuries. I can't stress this enough. If it was a child, you'd be running to the doctor; do the same for your dog.

Second, explain to the vet that you don't know what happened, but you need to know what could cause the injury. Could be injuries plural; you can't see internal damage. If he says the dog looks abused, get it in writing.

Third, even though you tried to discuss this with your roommates, it sounds like they're hiding something. I'm guessing the dog was kicked or thrown, depending on its weight/size/breed... by the way, what kind of dog is it? How much does it weigh? Are we talking little dog or big dog here?

You have a big decision to make - if they abused your dog, can you continue to live with them? Something to think about.

The vet information will help you decide what to do next. If the dog was definitely abused, you could go for legal action, but that might also make your living conditions interesting, where you're constantly waiting for revenge from them. If you decide to do nothing, then you have to figure out what to do with your dog when you're not at home. If they did this, you cannot allow yourself to ever trust them again.

I'm old and don't take to bullshit kindly; I can smell it all over their actions and comments, based on what you've said. And, if your dog is a little guy - yorkie, mini poodle, chihuahua, maltese, etc... then what they've done crosses the line with me; I'd file animal abuse charges.

But no matter what, take care of your dog first. Any vet with compassion will give you a free cursory exam and treatment plan, or at least discounted.