Thanks for the update. Several answers:
Fish oil- I would aim for about 6000-7000mg per day split into two doses. I would have to read the label to know how much volume to give, but this dose is much higher than the standard dose. This high dosage has been shown to reduce joint inflammation much more so than the lower dose used to improve skin health.
I would not panic at this time :-). I would suspect that she strained something when running or playing and is slow to recover. This is where the NSAIDs can be very helpful, but I know that you don't have any on hand. There are NO over-the-counter anti-inflammatories that I recommend for dogs. Aleve and Advil will actually kill your dog. Aspirin can be minimally helpful but can cause a very upset stomach. I would also slow her activity down to slow walking for 30-45 minutes daily and minimize running or wrestling with other dogs in the household.
Other more severe differentials include a spine infection, a blown disc in the back or severe bladder infection, so I would try to get her to your regular veterinarian sooner than later, but I would not panic and drive down there tomorrow.
When you said she is sitting sideways, I was able to picture it exactly. This is a common position for a dog with any hind-end pain, from the spine to the knees.
I am getting ready to log off for the evening, but I will be back tomorrow and will check in on Maddy.
I hope she is able to sleep well,