Enlarged heart/ sudden death

[giotta]

My beautiful dog Frida was a chihuahua and boston terrirer mix.  I rescued her from a shelter when she was about four months old and from that point on, for just over 10 years, she never left my side.  She was, as chihuahuas are, very loyal and attached to one person, which was me.  And I was to her.  I work from home, so she was with me every day, and she slept with me in the bed every night.  She was my soulmate.  
She had a cough, almost like a throat clearing maybe once a day for several months prior to her passing.  She was never sick and I contributed it to her barking, as she is extremely nervous.  I had my eye on her and I was trying holistic remedies.  But the last couple of days of her life she started to belly breathe.  I took her to the vet. She didn't act overly nervous in the 35 minute car ride, and she wagged her tail when I picked her up to bring her in.  But, she was an extremely nervous dog and she never went in cars (minus a couple of times).  When I entered the exam room and placed her on the table she collapsed.  The vet wisked her off, but came back 20 minutes later and said she didn't make it.  It is over a month now, and I still can't believe it.  Beside myself is putting it gently.  They did a post x-ray and saw she had a very enlarged heart.  I did not have an autopsy.  What I think happened is that she had the enlarged heart with no symptoms.  Then the minor cough started.  This cough wasn't every day until the last few days of her life, and it was extremely sporatic prior to that.  The symptoms rapidly progressed the last two days and it started to affect her breathing.  The excitement of going to the vet, I think, caused cardiac arrest.  I am completely devastated and it is only getting worse.  I feel so guilty that I didn't see the signs.  I coddled her and she did not go to the vet for check-ups because she was so scared of everything (and I previously had a very traumatic experience with vets with another dog).  She was looked after and pampered and I really focused on her mental state.  She seemed so healthy and I of course would take her to the vet if she was lethargic and/or had any obvious symptoms (not eating, diarreah, vomiting, etc.).  She had none. I thought she had irritated her throat from barking.  She was otherwise her complete self... running up and down 15 steep steps in my house at least 15-20 times per day... jumping on a fairly high bed at least 20 times a day and following me around, getting treats and being her normal affectionate funny self.  Very alert, energetic, and had her spark and zest for life.  She never panted, refused food or seemed out of sorts.  
The vet tried CPR on Frida and said she can only assume she passed due to a clot or stroke.  She did a post x-ray and saw she had a very enlarged heart, which was a complete shock to me.
Has anyone ever heard of such a thing.  It is surreal to me.  She was wagging her tail into the vet, and then she was gone.  I can't get past this and I miss her so so much it is unbearable.

If anyone has a similar experience and/or has/had a dog with an enlarged heart, I would appreciate any information on symptoms/prognosis.  It is all such a shock.