I doubt very much that you came on a visa on arrival. I suspect that you came on a Visa Exempt Stamp which is not any kind of a visa. Thai Immigration is currently allowing 1 x 60 day visa exempt entry plus a 30 day extension and then one more 60 day visa exempt entry plus a 7 day extension. But to be on the safe side, in your shoes, next time I would apply for a visa appropriate to my reason for stay.
There is no 7 day exension in Thaiiland. However, if you apply for a 30 day extension and itt is denied by Immigration, then you get a stamp in your passport advising that you must leave Thailand within 7 days - but it is not an extension of any description..