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Any time that I hear that a dog is suddenly falling over, I immediately think of a heart condition, particularly if the dog is not on heartworm prevention. If it happens more when she is excited like when you arrive home from work or when you take her out to play, you will want to speak with your vet about any heart conditions that might be present.
Also, with vestibular disease, the dog will present with stroke-like symptoms. They might walk in circles, fall over or hold their head an unnatural angle. If you suspect this, you can speak with your vet about medication to treat any inner ear infections.
I hope this has helped. Let me know if you have more questions.