Why do some doctors act disbelieving when i mention being allergic to a filler in certain meds? ie: some meds i can take in capsule, but not a tablet.

Uncommon: Many doctors may not believe your story because it is rare to be allergic to the fillers in medication although not impossible. If swallowing the medication is the issue then it is understandable capsules are easier to swallow. In your case, I would be focusing more on a potential reaction to the vegetable gum which binds the tablets together. The best thing is for you to consult an allergist.