I would consider swapping flea prevention and heartworm prevention brands and no longer give them on the same day. If you are using a flea med like Bio Spot this could be the culprit.
I would discuss this with your vet who may have good ideas on which meds to swap to.
I think the dog was 'over poisoned' and that even though he has tolerated it before he may be becoming sensitive to the meds being used.
Hope this helps you!
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