We wouldn't know for sure, but there are exceptions to your included text allowances.
See guide here Guide: Your Allowances - what is and isn't included
Maybe sent as an MMS ? Guide: Accidental MMS Sending and How to Avoid it
You need to contact O2.
All numbers are in this guide Guide: Coronavirus Community Help and Support
When you call, you will be asked for the reason you are calling.
Stay silent for 15 seconds and you will be taken to the keypad options.
Press 2 for Everything Else
Press 5 for More Options
Press 7 for Anything Else which will put you though to someone.
You can also try saying "Upgrade" or "Fraud" to be connected to an advisor.
8.00am is the best time to call if you can.
Track usage, upgrade and get support with the My O2 appDownload My O2