Burns On My Puppy's Back

[hellywelly4]

Yesterday I took my 5 month old puppy to the vet thinking he had just got a dry patch of skin on his back. They had to give him slight sedation and then shaved an area about 4" square which revealed a chemical-type burn. The vet asked if he had been near pine trees or varnish. I have racked my brains and can't think of any occasion when this has happened. The area wasn't irritating to him and didn't seem to bother him. The patch on his back looks like claw marks from a large animal.
Do any of you have any suggestions as to what might have caused this? I've got rhododendrons in pots on my patio at the back of the house - does their sap cause problems do you think although nothing on Google suggests this degree of burning, more poisoning by mouth.
Any suggestions gratefully received.

[Brinjal]

Any plant with white sap may cause that type of irritation. Oleanders hibiscus, poinsettia, euphorbias.

[chickensrock]

You have not had any lillys in the house in a vase?

[hellywelly4]

No. No lilies, none of the other plants Brinjal suggested; nothing at all that I can think of. I feel so guilty. He's much better now and the vet suggested I made him a little coat out of a sleeve, which I've done. He looks so cute, it comes down over his back, but doesn't cover his 'bits'. He was licking and biting his back, which was making it worse. Very strange. Thanks for your suggestions, anyway.

[lankeela]

Sounds to me more like an itchy reaction to a mite or flea infestation.

[hellywelly4]

No, the vet said not. Thanks anyway.