Potty trauma

James' is 21mnths so we decided to try a little bit of potty training. We have some time morning and night without nappy and see what happens. Needless to say we'll need to hire a carpet cleaner fairly soon. Anyway tonight he was sitting on the potty quite happily when he started to make his poo face. I carried on reading to him trying not to make a big thing out of it but he began to panic. I knelt down with him and tried to calm him down but he got really upset. We clapped and praised him once he was done but he just cried and ran away from the potty and tried to hide. I took him to flush everything down the loo and wash his hands which calmed him a bit but now he won't go anywhere near his potty. Has anyones lo behaved this way and what's the best way round it? The only thing I can think of is to stop the nappy free time and wait until he seems relaxed sitting on his potty again. I really wasn't expecting this cos he's usually so chilled about everything.