I'm sure everything will be fine. She probably just assumed you always take taxi from airport.
Btw if you are returning to airport from Jomtien you can buy bus ticket from the Jomtien bus station in advance, up to 3 days before you leave. I always do that rather than just turning up as buses may be fully booked. If they have a lot of people booking a particular bus they sometimes put on a second bus for that time.
Enjoy your trip!