Mini Doxie has immune system disease??

Our family adopted a very special boy 2 years ago, close to his 1st birthday he began vomiting blood and his poop was blood. Our 1st thought was parvo, after tests at the vet parvo was not the problem, instead she diagnosed him with an immune disease where his body attacks itself but the cause is unknown, we determind it to be because of his immunizations. Almost exactly a year later it happened again. We are lost and scared for our Vinny boy. We are beginning to wonder what we can do to prevent this or if this diagnosis is correct, we are afraid to take him to another vet in the case their new diagnosis will cause him harm instead of good. Another thing we noticed is it has happened both times during the beginning of allergy season. We are reaching out to see if anyone else has had this issue before or something similar to give us more insight into his health.