My 5 month year old cat died suddenly?

Well My cat died today. I was getting ready for my sisters high school graduation. When I noticed my cat on the bed just laying there. I didn't think much of it since I gave him a kiss a few minutes earlier and he looked at me. I though he was just being lazy :) but then things took a turn for the worse. I picked him up and he was limp. Faintly meowing. I decided to miss my sisters graduation and take him to the animal hospital. They told me they couldn't do anything he was in such bad shape. How can this happen he was fine last night. He cuddled next to me. I did notice Thursday when I got home he was swaying side to side and couldn't keep his balance. He had diarrhea. I just thought maybe he jumped down the wrong way. The next 2 days he seemed a little tired but still was his same old self. Jumping on me.:) He died in my arms. His green eyes turned black. Then I knew he was gone. How could this have happened. He was an inside only cat. Ate only dry cat food. Sometime I gave him tuna. Please someone help me. Too add to this pain. My dog got hit by a car and died last week. This is just too strange. :( please if you can give me any ideas what happened please tell me thank you.