Oh I see whats going on, the user group "Vendor" has a 1000 Limit and the user group "Registered User" has 550. Since you use to be a Vendor you had 1000 but now you only have 550 and your way over your limit. Unfortunately the limit of a particular person can't be changed, the limit is set in the user group (not individually).
You are going to have to either delete some old PM's to get below 550 or speak with Auto Guide. Can you delete some old PM's ?
Rob