Jimmyjammys - hope the allergy test goes okay and it's not the end of the world if your DC is allergic to nuts. MY DS is allergic to nuts, as well as other things such as egg yolks, egg whites, milk and sesame seeds, but negative for peanuts (which is the one you always hear about in the news).
It depends who you believe but one doctor told me my craving for humous and 4 pints of milk daily throughout my pregnancy may have made my DS more prone to allergy to both milk and sesame seeds (as they're in humous) BUT if that's true, then how did he get his nut allergy as I hate nuts and have always avoided them like the plague!
Just by giving your DS a tiny piece of nut, it's likely he already had an allergy of some sort. I've not heard anywhere that ust by having it makes you allergic. Surely you have to have an allery in the first place and if that substance is eaten / drunk, then it shows up - but not always!
So what I'm trying to say is please do not beat yourself up about 'giving' your DS a piece of nut andyou didn't cause the allergic reaction. I am grateful my son had anaphylaxia at aged 7 months after a pea sized bit of humous, as it meant we knew he was allergic to many things after testing and could avoid them completely.
Good luck, and please don't beat yourself up. At least now you know and are in the system for testing.