I’m worried about my 12 year old Jack Russell

My 12 year old Female Jack Russellwho is spade is being sick either once through the night or one early in the morning. She can bring up either food from the night before or bile. She refuses to eat dog food. I cook her chicken or beef and if she is having a good day I’ll mix it in with some Caesar fish dog food. She suffers tremors daily starting from her head and it radiates through her body turning her back legs into jelly she collapses to the floor. She remains conscious and with me. She is also suffering constipation and incontinence. She also suffers from arthritis! I love my dog and would really appreciate some help. She has been healthy all the time I have been lucky enough to have her in my life  

Hello! - there is a disease called Addisons that can present like this and if it were to be Addisons, then it ought be diagnosed pretty urgently, as this can be dangerous and lead to collapse.  I also wonder about fitting or neurological change - at 12, epilepsy is possible but there are other more likely causes with the signs as you describe them.  Then there are pancreatitis / liver disease, cancer and....  none of the above!   I feel that early presentation and a further history / tests to pinpoint a body system would probably be a great place to start.  Please will you let us know how you get o?