Hi, I am Dr. Peter and would like to help.
1- Any vomiting?
2- Is she on any medications?
3- Was the stiffness on all 4 legs?
Is it fair to say to she drinking more then usual?
Arch back is a sign of pain; which could be either related to abdominal pain or back pain. The fact you notice hardness in the abdomen, it could be related to abdominal pain. I normally would recommend pain killers like Buffered Aspirin, but, in this case, I will recommend not to give any Buffered Aspirin until she is seen by a vet and determines what is the cause of this pain. The aspirin could make certain abdominal conditions worst. I will recommend have her checked in the morning by your vet.
If you have additional questions I will be glad to answer them.