My suggestion would be to ring the ward where you had it done, and ask for their advice on pain levels etc. If they feel you need to be seen, then can then call you in. Tell them about your GP refusing to give you an appointment.
You can always go over the receptionist's head and ask to speak to the practice manager of course, but if they really are that short staffed and the practice manager has failed to get locum cover, then s/he needs to explain to you how your treatment is to be continued. If you have a named doctor, you could always write to them as well.