The place I bought it from don't use chemicals but then nursery they came from before might have. I heard that you only feed the new growth if a shop bought plant, is this true? If so then that is new growth as it didn't even have buds when I got it
What a beautiful rose! If it were me I would wait till these first blooms pass (cut those and put them on your table!) then feed the second blooms. I like to err on the side of "completely paranoid" though.