I am being charged for using VPC but I'm not

Hello.

Could you please share a detailed invoice image?
You can view your invoice by following the steps in the document below.
You need to check which items in your VPC are generating charges.
You can check the details by clicking the "+" sign on the invoice as shown below.
https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/getting-viewing-bill.html

I'm struggling to find out why am I being charged for VPC when 3 months back there wasn't any charge.

Since this did not occur three months ago, there may be a public IPv4 in your AWS account.
AWS started charging for public IPv4 from February 2024.
You can search for public IPv4 in your AWS account by following the steps on the AWS blog below, so please check it out.
https://aws.amazon.com/jp/blogs/aws/new-aws-public-ipv4-address-charge-public-ip-insights/

Please make sure if you have deleted NAT Gateways and Public IPS. AWS charges for those services in VPC.