Panting and trembling typically is a sign of severe pain. Your dog could have back pain from a slipped disc in her back, which can often cause these signs, and this could be treatable with steroids. Your dog could have internal pain from something like pancreatitis or liver disease, which could be treated with hospitalization and IV fluids and antibiotics. If your dog is not spayed, she could have an infected uterus. Basically, there are many things that could cause these signs, but these signs definitely mean that your dog will need to be seen by your veterinarian very soon, because your dog is uncomfortable. If your dog is still drinking and her gums are not pale, it is probably okay to wait until tomorrow if necessary, but I wouldn't wait much longer than that.
Best of luck... I hope your dog feels better soon.