were you paying for advertising through a group, or group land fees? could be a group you joined with some weird accounting roles deducting money, or marketplace advertising you forgot about that auto renewed.
But some groups can change your role without your consent even, they can just add you to the Accounting group and now you're paying for their advertising
maybe you had a few lindens to cover it every week for years, now it ran out ? just a guess! It's actually the most common thing for me, amongst my alts LOL
if it is a group thing, it's really easy to do without noticing. it's a tick box that says allow group contributions or something and after like 17 years in SL, I still don't know if I do it right
then the contributions are shared between everyone in that group, so for instance if you have 500 people in your group and you own a piece of group land, then each month, each person in the group pays 1 x 500th of the tier for that land
I filed a support ticket but I'm not holding my breath.