REST-API 3rd party resource model class
Where is your namespace?  If your are working in code bases that use namespaces, like Magento, you really should be namespacing your code as well.
The interface presented seems confusing to me.  Is it your expectation that the caller controls pagination through the result set?  If so, then how does the caller actually work with intermediate result sets, given that there is no interface to return the data set to the caller and your data in $this-&gt;data are overwritten with every iteration of load()?  How is caller supposed to assemble the results?  Also I would expect method names like getResultSet() to, you know, actually return a result set to the caller, not a string format used for pagination.  Why would this method need a public interface anyway?
I think you need make the determination as to whether this class should abstract away pagination activity from caller or not.  If you want the caller to have full control of pagination (and potentially assembling result sets based on pagination results), then I would think your class would need an interface that a) is more explicit around result set size and page numbers to be used b) actually return result set data to the caller.  This class likely does not need to maintain state the way you do now if you want caller to own pagination control.
You have a data mapping step in your load() function, where you change key for data set from data to items. I would suggest that this mapping function should exist in a separate method. In the class, so you can keep you mapping logic separate from your loading logic.