3 AM Attacks

About three weeks ago at 3 AM in the morning my mother, 84 years old, woke up with massive pain in her left arm.  We called 911 right away.  She still had time to put deoderant on and change her shirt because she did not want to embarass the paramedics. 

We all thought that it could be a heart attack or a stroke. So when the ambulance came, they took her to Queen of the Valley Hospital in West Coniva, California. Her EKG was alright.  After the X-Ray the doctor said she had an inflamation on her left arm.  My mother has arthritis in General. This is my question.....based on our general knowledge of arthritis, it comes gradually in with the time of a regular day. 

What happend that night?  Why all of a sudden a massive attack of pain?  Why all of a sudden at 3 AM in the morning when she is sleeping?  Isn't it supposed to rest while you are sleeping?

Thank you for your help!