Aww Kira don't be so hard on yourself, you are a wonderful mom
Maybe something happened while you were at work, a little disagreement , and it sounds like she was a little dehydrated, which will make them act that way. I know it is hard but maybe keep them separated when you are not there and let Puffy sleep with you like she always did. If you don't want to lock Chunk up at night maybe you could take Puffy in your bedroom with food and water and close the door so he has the run of the place and puffy has you. Spend extra time with Puffy and make sure she is getting enough water. She will be ok, it probably is just going to take some time, both of you, heck all three of you have had a lot to deal with.
Hugs

Just wanted to add, yes they most definitely know when we are upset and it upsets them, they only know something is wrong. I was so upset and depressed when the girls were sick as kittens and my vet kept telling me that I was making it worse, to be more positive. Sometimes I would go sit in the car and cry so they wouldn't see, then paste a happy face on. Of course they still knew, they are very tuned in to us.
Try to make things as normal as possible for Puffy, she is probably feeling like you are, totally overwhelmed. I sure wish we could learn cat talk so we could communicate