Almost certainly a second project. On paid plans every project runs its own compute instance, billed per hour. Pro includes $10 of compute credits, which covers one project on the smallest (Micro) size. A second project, even an empty test one Lovable created, adds roughly another $10 a month.
The spend cap only limits usage overages (egress, storage, MAU...). Compute, and add-ons like PITR, custom domain or IPv4, are always billed.
Check Organization > Projects. Delete (or move to a free org) anything you don't need. Paused projects don't accrue compute.