Mysterious Guppy Death

So maybe it wont be so mysterious to you guys.
 
10 gallon tank. (Was) 3 guppies, 2 female, 1 male and 2 rasboras. Male has been in the tank at least 3-4 months. Females and Rasboras 5-6 months. Tank is well established and parameters are fine. 
 
I found my guppy dead this morning. Completely (almost creepily) extremely inverted. Never seen that with any of my dead fish before. The thing that weird-ed me out was I saw ZERO signs of any sickness or strange behavior from this guppy. She was always healthy and the stronger (larger) of the two females. Was eating as of I believe yesterday. When I bought the male I obviously came in wanting to possibly experiment with pregnancy/fry. However, as time ticked by there was no pregnant females (strange)...so I sort of just gave up on the idea and moved on with my life. The last time I looked at this guppy I actually noticed she looked a bit larger in the belly but I figured it was just food as it usually is.  After seeing her this morning, as you can imagine, I was shocked to see her as in the attached photos. There was also a slit in the belly and all the other fish were starting to nibble her away. (Also a behavior I hadn't seen in my tank when I had fish die before). Was she pregnant? How do you propose this happened so abruptly?
 
Thoughts??
 
 
 
 

Could you post the actual numbers from your tests, please?
 
Unless we can rule out water quality issues, we can't really help

Not really going to be of any help but i had a similar issue a while back that went undiagnosed,
 
It mainly seemed to effect females and often came on quickly causing them to develop hunches and die over a short period, i wonder if its a similar issue? hmmmmmm
 
Although looking at the pictures looks like a pretty standard dead guppy to me

I mean, I test the water with a strip test and the ppm was basically all in the safe range. I've brought in water samples to my local fish store before (after doing a strip test with similar results after a fish death) and they've told me the water quality shouldn't have been the issue. If you want approximations:
 
Ammonia- 0.3ppm
Nitrate: 20 ppm
Nitrite: 0ppm
Hardness: 150-200 ppm
Chlorine: 0
Alkalinity: 120
pH:7
 
I'm fine with speculation. Just opening up to discussion. Just wondering if anyone noticed anything particular from the photos/description.
 
Is the slight protrusion on the underside just normally how females look underneath pregnant or not?

Ammonia should be zero. Strip testers are not very accurate so get an API test kit. (or equivalent in US). She does not look gravid normally gravid females are black where the fry develop. What is the age of the fish approximately.? Don't let the other fish eat any part of the dead one. Do a P.M. on the fish and see if there is anything unusual inside her.