vet bill

Starting 3/2007 I took a dog to a vet,made a payment that day,continued phone consults over past year. I just got a bill for some charges that I never once got a bill for and the charges are for March 2007. The last bill I got was in Oct 2007, still no indication of owing more than for the phone consult in Oct. There was never any mention that any unpaid balances would have a finance charge applied to them. I knew we had a balance (that we've been working on paying) of just over $200. A bill arrived today, 1st one since Oct 2007, for $675.47 including finance charge $79.93. No indication of finance rate, is this legal? Can they all of a sudden bill me for something that they never billed me for before especially when it is going back 11 months? I am in Michigan. Thanks