Sorry about your Mama guppy! Did you do the water change gradually? When my mom had fish, she would syphon the yuck out of the bottom with tubing, and only remove maybe half of the water from the tank. A sudden change in water temp or PH isn't good. Do you use litmus paper to test the PH in the tank?

I don't have guppies now, but I use to.  It was so much fun when they had little ones.  They were SO tiny!!  I always netted them off in their own area of the tank so that the other fish wouldn't eat them.  A few walways died, but I managed to get some grown.

I never changed anything in the tank while they were small.  I was too scared to...lol. 

The change in water temperature could have killed the mama guppie.  Just last week we changed out the water in our tank and lost 3 fish.  One was a huge Angelfish.  Prior to this, we only changed about 25% of the water once every month.  They had a really nice eco-system going, but hubby said it was time to do a thorough cleaning....so he did.....lol

I guess we need to learn more about how and when to do thorough cleanings...lol.  Really, if you have the underwater vac, then they probably aren't necessary.  You can just get by with changing out a certain percentage of the water.  That worked for us for over a year.

Once your tank sets up its onw eco-system, which takes about 6 weeks, then you really don't have to maintain them a whole lot.

Guppies are my favorite. They are native Mexican stream fish and are terribly sensitive to change in temperature/ammonia levels. Get a suction held ammonia reading tag for the tank, so you can see how the water goes. It looks like a 1in. color wheel.
Use Cycle by Nutrafin, a dose in the bio-filter, and a dose in the water, every week
[1 capful].
Use both Ammolock, and Stress coat in the tap water, before adding to tank. I never was good at water changes. The suction off the bottom is important, but I just add conditioned water, and bacteria, when the level is low, no changing here. I also have a couple ornaments, but no rocks or live plants[collect ammonia, and disintegrate to cause ammonia].
If it is any consolation, the females usually die after they give birth. I had hardier ones when I was a kid, but these days, they are not bred to reproduce. The gups that I had as a kid looked tough, like a Mollie. The males were small, the females were big and mean. The males would all chase the big old girls. These delicate ones are bred in Thailand and other places...The fancier, the older, and more delicate, the fish. The fin is an indication of age. The back is straighter on young ones, and the tail develops as they age, so the prettiest ones are fairly old. The best way to pick them is to look for lively, hungry, fat, horny ones...