Unfortunately the forum is now largely a referral tool to other Optus contact points.
You should be using the settings on your phone to track usage locally. If its Android you can set usage warnings and cutoffs and set the data to reset each month. If its iPhone unfortunately Apple don't encourage tracking data but you can at least reset the data usage counter at the start of each month and manually check it as required.
What does your phone show now?
Hopefully Optus can get the issue sorted but if you do decide to head elsewhere I can recommend Boost Mobile. The 12 month plan for $300 gives you 240Gb. That's effectively 20Gb a month for $25 (use more some months and less others) which is probably less that you are currently paying.
Optus has a similarly good plan prepaid but its for new services only. So you either get a new number or will need to port out to a cheap $5 SIM with another provider (Amaysim?) and then port back in if you want to keep your number (no idea why Optus make their own customers jump through all those hoops)
I have also noticed a significant increase in my reported usage.
Increased my plan cap. and agreed to unlimited data days. Even so they have charged me $120 over twelve hours.
Either their systems are bugged or they're trying to overcharge us. Will probably change carriers.