Tree nuts are one of the top allergens and most common food sensitivities. read more here
If you continue to eat something that you have an allergy or sensitivity to, it is very difficult for your gut to heal and for your immune system to deactivate. Additionally, the fiber in nuts and seeds can be difficult to digest, particularly almonds, pistachios and hazelnuts which is an additional way that some people can be sensitive to them.
I find soaking nuts helps me digest them. After soaking I put on a cookie sheet in the oven for 12 - 24 hours depending on the nut. Here is a blog site with soaking instructions.