Dog died after coughing.

Hi,
My name is Tania. I had a Maltese/Shih tzu 12 years old. She had a stage one heart murmur, a melanoma (4mm), multiple warts (4) on her back and had one on her eyelid, a throat infection (diagnosed on 19 November 2012) she was on 50mg Enrofloxacin (1 a day) and had a fever, she hurt her leg on the 26 November 2012 took her to emergency vet and they gave 2 injections Dexadreson 2mg/ml and Niramine 50(MI) thinking it was a sting, she stopped eating and drinking and started vomiting yellow n green foam.
Took her to our vet the next morning (27 November 2012) for she was crying in pain all night and he found it was a cracked front right leg at the funny bone. Was hoping was no nerve damage. She was given an injection of steroids and injection for the pain.
28 November 2012 she seemed like she was getting better, was drinking water when I picked her up and put her at the bowl. She wagged her tail and walked a little. Then late that night she got a cough (I took a video of it that I'll send to your photos email under "SHY PIRAINO") and thought might be the throat infection because she didn't finish her medication and we had the first appointment in the morning. She vomited a few times yellow foam, her heart beat n breathing was pretty fast and she stopped drinking again.
I didn't have any way of going to the vet at that time.
29 November 2012 9am she past away, she just rolled on her side and blood poured out.
The vet thinks she died of an asthma attack and I'm worried if I could have taken her to the vet the night before she would still be here.
The cough was like she had a bubble in her throat and couldn't cough it out.
I would just like to know what you think the cough may have been and what you think may have killed her.
I know there can be multiple reasons but I feel like its my fault and she could have been saved.
The vet couldn't believe she past. At least she dead with the people she loves and loves her most around her. I never left her side even when she tried to be alone. I tried to make her as comfortable as possible for she couldn't walk and put her in her favorite spot in the house. I didn't know she was going to die but she looked so depressed I was just trying to make her feel better with kisses and cuddles.

Thank you so much for taking the time to reply.

We had a feeling it wasn't an asthma attack and even though we wont know exactly what killed her, you have really helped give us some peace of mind.

Every question we had you have cleared up for us, so thank you very much.

Sincerely
Tania & family :)