If you are staying in a hotel, the hotel would have registered the TM30 online. If you are in private accommodation then you may be in trouble. You should not leave the extension until the last day. Most offices let you do it anytime in the last 30 days. Jomtien will only allow the last 7 days.
If you use one of the many border-run travel agencies then they virtually guarantee you can come straight back in as they have 'special' arrangements with border staff