will be helpful for those in need It means one of your Models (classes which the Code-First turns into actual tables in your database) has changed. This can be a change on one of the fields, like adding a key attribute, or adding a new field to the Model.
will help you Doh! moment, turns out the service wasnt using the same connectionstring as the other app. the reason the databases didnt look the same must be because earlier in development i started the main app with no connectionstring aswell, so it provisioned a local instance database for itself to use. Then later when i was trying to use the service, it was trying to access the same database from earlier, and the model changed significantly since then.
The model backing the 'POSContext' context has changed since the database was created