Can I apply for a 30-day visa extension in Thailand after being warned about my stay?

Hello! US passport here. I have a question about applying for the 30 day visa extension to the visa exemption I received when I arrived by air yesterday. I planned to stay 45 days and then go to Sri Lanka (flight already booked). They held me at immigration on suspicion of working in the country and pulled up all my times leaving and coming -- even on my former passport -- and told me I would have to leave after the 30 days and couldn't get the extension. Then forced me to book a flight in 30 days out of the country. The official warned me to get the 60 day visa next time before arriving (which by the way I tried to do but the consulate general was on holiday) and wrote the flight info for the one she made me book as well as my old passport number on my departure card.

What I'm wondering is, I would like to cancel the flight she made me book and apply for the extension as originally planned. Can I still go ahead and do this or do they make some note in the computer and I'm going to get a red stamp or something if I attempt to get the extension? Thanks in advance!