Purchasing Vendor Limits

We are on 5.1 and just came across a situation that has never occurred
before. We issued a PO to a vendor a few weeks ago that supposedly put
us over a credit limit that we didn't know we had. We had a machine
break down today and had to order more parts from this vendor and they
refuse to sent the parts because they claim we are over the credit
limit. My question is, there is no where on the vendor file or PO
anywhere that mentions a vendor credit limit. Other than putting a
comment on the vendor file that doesn't show anywhere how is anyone
handling vendor credit limits to you. Thanks for any suggestions.