Back leg spasms

Every time my 15yo cat tries to lay down her back side starts shaking like she having some time of muscle spasm. Then this morning she was rolling on the ground and it was like she was unable to get her footing to stand up. Is this something I need to go see the vet for, or is this pretty normal.

Hello!  This sounds as though it could be either painful or neurological and either of these two things would require attention.  You don't say how long it has been going on - if it's a very new symptom, or more severe than previously seen,  then it may be worth getting triage (eg on the phone to the emergency vets) tonight as depending on the particulars they may not be happy to wait until the morning.  Over recent years it has become increasingly apparent that cats mask / hide arthritic pain very well and sometimes, if no other abnormalities are found, a trial treatment with pain relief is recommended.  However, blood clots, neurological abnormalities, pancreatitis, etc may be on your vets list.  This is a wide best of possibilities so triage, with an examination and chat at the appropriate time following it, would help to narrow down the likely causes of the signs.