Previous posts on this topic include a summary by u/amp1212 which ties back to this post by u/michellesabrina.

Unfortunately, I am not able to directly provide any situations where an allergic reaction was linked to poison; however, I can provide some details through the work of a sixth century 'culinary physician', Anthimus, which would suggest that they had some idea that certain foods just inherently did not agree with certain individuals.

Let's start with look at a common theme throughout his text, which is that of the 'healthy' people and 'sick' people. The Latin he uses here is typically just sanus and infirmis, which unfortunately does not provide us with too much extra detail. However, we get the sense that as he is talking about the infirmi, Anthimus is grouping together individuals who have some sort of faulty internal organ, as he would have no other capacity for understanding issues with digestion (he refers on a few occasions to the liver and kidneys being faulty, for instance). He does use this distinction fairly often, when suggesting whom a given dish would be suitable for.

I'll provide a couple of examples here where he makes it clear that it is ill-advised for certain people to consume certain foods. The first can be found in section 61 of his text, discussing garlic. As mentioned earlier, he associates issues with garlic to a defective kidney, and as a result suggests people in this circumstance use it quite sparingly. Unfortunately he doesn't go into detail about what issues this would cause, so we cannot say whether he is relating this, for instance, to indigestion which may be involved with IBS, or whether he is associating garlic consumption to another ailment. In any case, we have a clearer picture with dairy products, which should not be surprising! Anthimus begins his discussion of milk in section 76 of his text where he immediately begins discussing how milk, consumed by 'unhealthy' people, will curdle in the liver and stomach, and is 'extremely harmful' when consumed. This comes up again in section 78, speaking of purposefully curdled milk (cf. cottage cheese?), and again in section 79 when he discusses the harms of cheese. So we can tell that Anthimus was aware that certain foods would cause a certain reaction in some. In fact, perhaps about a quarter of his sections discuss the healthy/unhealthy dichotomy, suggesting that he was well aware of how certain foods could be harmful to certain individuals.

Finally, I'll provide one last excerpt from Anthimus which does, to a degree, address the poisoning issue of your question. In section 25, Anthimus discusses turtle doves, especially those caught in the wild. He advises against this, as there is the potential that these birds have recently eaten a plant which he calls 'hellebore'. He then goes on to say (regarding the hellebore-fed turtle doves), 'So if someone catches a wild turtle dove which happens to have eaten of this plant, then that person undergoes great danger if he eats this bird. For it brings a man extremely close to death, so much so that anyone in ignorance might think that he had been given poison by someone.' So in a way, Anthimus tells us that it is possible that a reaction to food could have been confused for a poisoning. At the same time, he raises the point that this is only among those who are ignorant to the food and its reaction in those who consume it.

So, I hope that these excerpts of Anthimus can begin to fill out this picture for you, that people in late Antiquity were at least aware of the kinds of reactions to food which we now associate with food sensitivities and food allergies. Also, we might theorize that it was not outside the realm of possibility that reactions to food could have been mistaken for poisonings; however, Anthimus suggests that this would be out of ignorance of that food, and that physicians might not make this wrongful association.

Edit: I have corrected the declension of sanus.

This previous question on a post by /u/mjolle questions the actual veracity of one of my favorite Viking sagas that has been interpreted as the native North American skrælings attacking as revenge for their perceived poisoning from dairy products gifted by the vikings.
As mentioned, the lactose intolerance part is a modern interpretation on some bits of the saga that was written down 250 years after the fact.