Why does my dog keep sneezing?

It all started yesterday when my dog was eating these thorny weeds that were sticking to his fur. (The weeds look like wheat?) Then, he started to gag and sneeze vigorously and blood came out through his mouth or his nose. i'm not sure whether it was the weed that was lodged in his throat or if he has allergies. I looked up online about possible causes and tumors, allergies, or something lodged in his nose came up. Should I go to a vet? any home remedies?