I had 4 visa exempt entries to Thailand over a 2 year period, non back to back, longest 28 days, previous 3 for 10-14 days.
I applied for ED visa (approved in Sydney, Aus), and was hassled at the border in a similar fashion to yourself. I was allowed in however, and am currently on my 2nd extension.
For the extension, expect to be tested on your thai language skills, and as my next extension will be after 9 months of learning, I was told "we will be testing you harder next time.