My Guppy Randomly Died.

I brought some new fish today (4 neon tetras, 1 catfish and 2 guppies) and they are all doing well except one of my guppies.
She was always swimming at the top and just acted off, then she kept swimming up to the filter part and it kept sucking her towards the gaps... she was able to swim away on her own as she did several times and i did try moving her away but she kept purposely going back to the place and allowing herself to remain somewhat 'stuck' there... I noticed it was odd as she was the only one doing it.
Later on I noticed she was stuck there for a while, so i moved her away and she just sort of floated around the tank spinning around but still alive.
So i tried to keep her steady and then it looked as though she died so i filled a  cup up with the aquarium water and put her inside, after 10 minutes it seemed as if she had died so i went to flush her down the toilet and when i help her and poked her a little to ensure she was dead she suddenly twitched and wobbled a little so immediately put her back in the cup and she wobbled a little but then floated again and now she looked lifeless
 
I am completely confused as to what has happened
Unless maybe she was ill from the tank she was in at the pet store i have no idea but all the other fish i have purchased seem fine even one of the guppies who were in the same tank as her
 
She was a black/blue female guppy and her friend was an orange guppy so they were from the same litter.
 
Little confused. Any ideas? Did i do something wrong?
 
The image below was what she was like during the whole time in the tank, she wasn't stuck she just chose to be sucked and stayed like that...

Well I'd say she was probably stressed. Was there anything in your tank prior to adding your new fish? Is it cycled? And how big is the tank?

It is possible that she had something wrong with her, but I'd say more than likely the death was probably stress related.