Edward is correct. And he has the credentials to back it up. (See his profile description)
VersionControlServer.QueryHistory is the method you need to use. There are several ways to use it and I'm only describing one below which assumes that the server path of that item is what is important to you...
First, you need the server path of the Item:
string serverPath = Item.ServerItem;
Next, if you don't already have a VersionControlServer object instantiated, you can get one from your TeamProject like this:
VersionControlServer VCServer = (VersionControlServer)this.TeamProject.Store.TeamProjectCollection.GetService(typeof(VersionControlServer));
Use the VersionControlServer QueryHistory(string, boolean) method to get other changesets associated with that server path: