See
http://www.limewire.com/english/content/faq.shtml#Pro (: How much does LimeWire cost?) as it specifies, LW Pro is for 6 months support & free updates only. Almost all software programs are the same. After a period of 6 to 12 months, you are offered a discount to continue with a newer version & support, but if you don't then you're stuck with what you have. After 6 mths your name drops off the database. You were offered a discount to continue but you obviously decided not to.
I presume you have a copy of your installer. I also presume you bought it for specifically $18.88. If it cost you more then you didn't buy it from the official site.
Membership? - **WARNING**-Download p2p Clients ONLY Thru Official Sites (click on link)
BTW if you don't wish to purchase pro again, then you can get LW Basic for free from here
http://www.limewire.com/english/content/download.shtml