There are many things that can cause a sudden onset of limping like this. The most common of which is a simple sprain. She may have injured while playing, or taking a bad step when jumping off of a bed or couch, and now it is inflammed. Since it has only been this way for one day, this would be at the top of the list of possibilities. I recommend that you take her into the veterinarian for a check-up.
The vet can examine her to make sure that nothing is broken and will look for any obvious signs of a puncture wound (that could be causing infection), or of an infectious disease that causes fever and swelling of the joints (like Lyme disease, or Ehrlichiosis). If all looks okay, then the vet will prescribe an appropriate nonsteroidal anti-inflammatory for your dog like Rimadyl or Derramax. This is usually administered for about 5 days. If the swelling and limping subsides.....GREAT! If not, then the dog returns to the vet for X-rays of the affected joint to look for signs of arthritis, fracture, or bone cancer.
It is important that you do not give anymore aspirin and that you be sure to let your vet know that you have given aspirin already. This medication can interfere with some of the medications that the doctor would want to prescribe for pain.
I am also attaching a client information handout that I use in my practice that discusses the causes of lameness in dogs in more detail. I hope that you find it useful.Click Here
Because you say that she is not herself, and that she is really having trouble using this leg, I would not wait until Monday to have this addressed. You should take her to the emergency clinic today so that they can get her started on some effective pain control to get her more comfortable right away.
I hope that this information is of help to you, and I wish you the best of luck with your dog. Please let me know if I can be of further assistance.