I have to say that I am very worried about her.
The level of pain you are reporting just is a major red flag here. As it is towards the back half of her body, we'd hopefully be able to rule out pancreatitis and gut twists. As well, if foreign bodies can be ruled out, then we'd again be thinking about injury, possible spinal arthritis (if she isn't young), or even slipped disc.
Now aspirin is only a mild pain relief (and needs to be used with care if we have possible gut issues or a possible bleed anywhere) and we'd not expect it to make much difference with a dog in as much pain as you have noted. In this case, we really need your lass to her vet urgently or them out for a home visit to her. We need her on strong injectable pain relief to get her comfortable so that they can then pinpoint which issue is present. And I would note that if you think she looks to be sore in her back at all or is struggling with her back legs at all; then a disc is a worry and that is an emergency situation for them (as they can progress to paralysis in some dogs).
With all this in mind, this is unfortunately not sounding like a situation you can manage at home. So, we'd really want to take the above steps to address her pain and make sure we give her the best chance of recovering from this.
In this situation, it would be prudent to get your wee one to the emergency vet. To find your local ER veterinary clinic, you can check http://www.vetlocator.com/
Please take care,