I am naughty, and usually take a double dosage of an anti biotic when I start a course to get more in my system quickly.
However, I think that you should notice a difference within the first 24 hours, and a definate improvement within 36 hours of commencement.
I would make another appt for this afternoon with GP if I was you, and just see if it improves by then. If it does then you can always cancel the appt.
Certainly don't think you should go to the ER unless the pain is so unbearable, as the hospital will see that you have only just be put on anti biotics, and will probably do nothing more at this stage.