Something afoot?

Oscar, my Beardie, was very poorly on Saturday morning. I woke up to find him quite altered. He'd been sick several times and Tess - unusually - had not attempted to clear it up. Closer inspection revealed it was undigested dinner from the night before. I'd fed both dogs the same thing - some Naturediet from the same packet and some sliced cooked chicken that I'd bought for the kids pack lunches but not needed.
Both dogs had eaten the same food, yet Tess was fine. Both dogs had drunk from the same bowl and pond.
Oscar went out and started straining to go to the loo. No matter how hard he tried nothing much emerged, but what did was tinged with blood. He took himself to his dug-out hole under a bush and lay down. He seemed very depressed and limp. He was so still I was afraid he was dead.
I phoned our new vet and the receptionist could sense the panic in my voice. There was a smell about Oscar that reminded me of old Sally when she had parvo. The vets were lovely - come immediately they urged, even though surgery was over.
Oscar was not his usual self at the vets. He kept sinking to the floor to sleep. No paws on anyone's shoulders. A little flick of the tail if anyone talked to him, but he looked terrible.
His temperature was high, he had a lot of gas but no obvious painful blockages.
The vet gave him an injection to stop him being sick and an antibiotic jab. Plus I went home with all the rehydration tools in case he stopped drinking.
She said that while he was inoculated it was best to be cautious and consider it might be parvo and not let him mix with other dogs just in case he was contagious.
It was time to take him home, watch him closely. Ring the on-call vet if he got worse and bring him back on Monday for more tests.
Poor old Oscar, I'd never seen him so quiet.
While I was bathing him to get rid of the stink I noticed something odd about one of his paws. It was a front paw. At first I thought he'd stepped in some of the shampoo so I tried to wash it off. But no matter how much I rinsed I couldn't shift it.
When I looked closer there was a sharp edge to this patch of gooey stuff and wrapped around a bit of the hair inbetween the pads was something that looked almost glue-like.
I spent about half an hour after the bath using paper towels to scrape the stuff out of Oscar's pad. I remain completely baffled as to what it was. It had no smell but a very odd texture indeed. It made his pad suddenly shiny and smooth like it had made it waterproof.
His pad isn't sore now its gone and has returned to normal.
Yesterday at the vets his temperature was perfect and he was back to giving Beardie hugs.
Could he have absorbed some strange chemical via his pad? What was it and where on earth did he get it? Did he try to lick it off and was it that that made him sick?
Best guesses please?
It was clear and gel-like. It wasn't so sticky that it had collected gravel from the ground or stuck to Oscar's copious fluffy leg hair. But it seemed to have bonded so very strongly with Oscar's paws. It could only be removed by scraping with a fingernail.
He'd not been walked on pavements, just common land. Tess's paws are fine.
Weird! But a huge relief to have him back to his old self.
He's such a clown about taking his tablets. For such a big dog he does eat remarkably delicately. Oscar can easily detect a tiny tablet fragment in a lump of cream cheese wrapped up in a piece of ham. Tess has her placebo parcel swallowed as soon as I give it her, but Oscar is still chewing minutes later and trying to catch an ejected pill shard before Tess gets to it is an art form!