My first dog was a pit mix who had hip displasia. It showed up when he was two with the same kind of symptoms you're noticing in your dog. By the time he was 5, the related arthritis was severe enough to require daily rimadyl which made a tremendous difference in his life. A few years later, we switched him to deramaxx when it became available and WOW. That stuff is wonderful.
Hip displasia is something you'll need your vet to diagnose. In very severe cases, surgery is necessary. In all cases, the inflammation needs to be managed on a daily basis. There's no reason she can't still be an athletic dog - maybe not mountain biking - but take some time to slowly warm up and slowly cool down. As ginger notes above, swimming is a perfect exercise for a dog (or human!) with any orthopedic problem.
For now, she needs rest, a vet exam and probably an anti-inflammatory medication like rimadyl or deramaxx. If there isn't noticeable improvement in a week, it's x-ray time.
I mispelled hip displasia at one point, in my above post. The correct spelling is the second way that I spelled it.
By the way, if after she's been checked out, had appropriate treatment or whatever, and has rested to heal any possible injuries etc, -she needs some physiotherapy...then an excellent physio. for dogs is swimming. You would have to ask the vet's advice about this first of course.
But swimming is gentle yet helps to build back strength in muscles and joints. The dog's weight is supported by the water, so it is not as stressful as walking or running. Racehorses, in rehab. after injuries, are given swimming physio. to build back their strength. It works the same for dogs.
She needs to be checked out by a vet. She sounds a little too young to have developed arthritis. This sounds more like an injury of some kind. Or she may have some trauma or injury to her lower spine or hips. She is likely to be in some pain, so get her checked out as soon as you can.
Another thought...has she been bitten by a Tick in the last few weeks or months? Does she walk in grass, undergrowth, woodlands, fields? Have you had to get a feeding Tick off her at any time recently? If the answer is yes, it could be that she needs testing for Lyme disease. If positive, she would need a long course of antibiotics.
Depending on what is wrong with her, she may need extensive rest with only very gentle movement for a good few weeks. During this time it's important to try and keep the weight off her, as extra weight will put pressure on her joints. The less she exercises, the less food she needs. But make sure her food is a very high quality. She'll need good nutrition, just not so much as when she was exercising hard, running.
I do hope you find out what's wrong with her, and she gets better very soon. If she is treated properly, she may well be back to her normal athletic self in a few weeks. Let's hope so.
I think you are going to have to have veterinary assistance, if this persists beyond a few days. If it is just a matter of overworking her after a long winter layoff, the problem should be temporary.
If it is more than that, it is a hard thing to assess long-distance. Sprains and strains are usually unilateral. If the problem is bilateral, well, I guess she could have injured her back while playing. If that is what it is, it could be either minor or serious.
I would also want hip dysppasia to be ruled out, if she continues to be in discomfort. Around the age of two is when that can show up sometimes. Actually, hip dysplasia can show up at any age, but early adulthood is not an uncommon time for it to rear its ugly head.
In the meantime, I would rest her and give supportive care as you have been doing. You might give a baby aspirin (with food) and see if that helps, but I would have her off the aspirin when I took her to the vet. I'm sorry for the situation, and good luck.