Help! Accidental overstay?

First, I know this is totally our fault, we should have checked the stamp, and we didn't.

Situation: we left home (New Zealand) at the start of April and were granted a multi-entry tourist visa (METV)about 2 weeks before we left. This is the one where you can stay up to 60 days at a time. We arrived in Bangkok at the start of April, so naturally assumed we had 60 days to leave. We have been holidaying in Hua Hin.

We want to do an extension so were going to head into an office this week (it has been 50ish days) - we just looked at our passports and the stamp says our entry date and then only 30 days later the exit date (at the start of May). But on the page prior it has our full page for the METV and the date it was granted. Have we accidentally overstayed? Was this a stamp error? Is there any way to explain the situation?

I feel like we made the effort of going through the full formal visa process and paying all the full fees (for all 5 of our family members) and now we face an absolutely massive fine if we've overstayed 😭