I've been using Spotify with my NUS discount for a while... I've just seen that last month I was charged £9.99 instead of £4.99 (at that point my discount card was still valid) and I had already extended my NUS extra card to be covered until 2015.
Not happy about the lack of contact from spotify either to warn me about any of this!
You will need to get in touch with the customer services team directly using the online contact form and they will be able to assist you further with this. Spotify NUS discount is only valid for 12 payments on each card number, so if you extended but have the same card number, Spotify will still only give you 12 months of discounted price.
If you get an automated email reply back directing you to the community or help pages, you need to reply directly to that email (even if its from no-reply) and one of the customer services agents will get back to you as soon as possible.