No one advised via this forum on billing matters. Possibly you regular bill was already on the way when you registered? They get printed 5 days or so early. If you want to sort it then its several hours wait on the phone or online usually (if you can get through). Otherwise I'd just pay it and wait to see if the next bill is correct.
Optus will either add the credit soon (in which case you payment will just become a credit to pay a future bill in 4 months) or you can get it sorted out and re-credited later on when things are more normal. Either way you'll end up with 3 months free in the end.