Some of this is not a training problem, and treating it like one wastes time your dog may not have. If they are distressed when you leave, if they growl or guard their food, their toys or you, if they are frightened on the lead, or if anything about them changes suddenly at any age, contact your vet the same day. A tip from a website, ours included, is not the answer. If it cannot wait, contact your nearest emergency vet now.
With a Chihuahua, watch their breathing, their energy and their eyes. A honking cough can be their windpipe. A young dog who goes weak, wobbly or very sleepy may have low blood sugar: contact your vet now. Bloat (GDV, a twisted stomach) needs the same speed: a swollen, tight belly, retching with nothing coming up, sudden restlessness or drooling. It can happen in any dog, though it is most common in large, deep-chested breeds. Squinting or a watering eye needs looking at the same day. Circling or pressing their head against a wall is never a quirk. In an older Chihuahua, pacing at night can be the start of Cognitive Dysfunction Syndrome (dementia in older pets).
They'll keep walking on their own four feet. If they start honking like a goose, tell your vet.