Rat dead with no warning and few clues

Question
At the beginning of the year, I had three rats.  A couple of weeks ago, I lost my oldest to mammary tumors.  She left behind two cagemates, roughly one year old.  I came home this evening to find one of them dead, and I have very little idea why.  
*She had a piece of bedding in her mouth, which makes me wonder if she was eating it and choked on it.  Her jaw was too stiff for me to pry open and see.
*She had been looking bulkier than I remembered, but I thought it may have been happening for a while and that I may have just missed her weight gain from paying so much attention to the first one we lost while she was sick.
*I can't remember how long it had been since I saw her alive.  I either gave her and our remaining one a snack early this afternoon or I saw her last night while checking their food and water.  In either case, she acted perfectly normal the last time I saw her.
*She may or may not have had sniffles; in any case, she never seemed sick at all.
*I went to a pet store yesterday afternoon, so I suppose I could have brought some germs home with me, but again, she never acted sick.

I'm looking for a most likely cause of death, but I don't know if I can afford a necropsy right now.  I'm also trying to guess how long she had been dead.  Rigor mortis appears to have set in pretty well, and her claws had a deep red tinge to them--I was checking for scratches on the other rat because I thought she had blood on them.  There was no noticeable odor.  She was fairly cool to the touch.

I'm now nervous the other one is just going to drop dead on me too, either because there's a hidden disease or from heartbreak.  Any ideas?

Answer
I am so sorry for your loss. Sounds liked she was in pain, which is why she was eating her litter.
Its so hard to say what went wrong but chances are she had an internal mass, esp if she had some fast weight gain.  I have lost alot of rats suddenly and cause of death was either an internal mass, brain aneurysm or heart attack.  Not long ago I lost my beloved rat, Ben. One day I came in the room and there was blood all over the cage. I almost passed out I was so scared for him. He was FINE but he was also very bloated.  The vet and I decided that even if he did have a mass, what can we do, we surely could not operate, not that type of surgery on a rat especially.  He was FINE for two days, eating, getting around ok etc....and the third day after the bleeding he started to act needy, wanting help, which was NOT like him.  I took him to the vet because I knew things were not right and by the time we got there, my poor boy was in a frenzy with pain, screeching, clawing up my body, I was so upset I yelled for the vet to sedate him so we could get films.  Sure enough he had a large mass in his colon, which explained the blood in his litter box and all over the cage. There was some blood on his rectum so I knew it had to be internal that day, but since he was OK I could not put him down that day.  We never let him wake up from the sedation and put him deep under anesthesia and let him go to the rainbow bridge.  I wish I could have put him down before he had to endure that pain but we just never know what is going on with these rats.  I had a necropsy done, and my vet never charges me, as we see it as a learning tool for both of us and it benefits other pet rats as well.  The charges come in if we send tissue out for testing but my vet can see things that are visible with the naked eye such as irregular organs and tumors etc... It was for sure cancer but of course without a biopsy we dont know what kind, but it doesnt matter, he is gone now and we both have learned what signs to note when rats have colon cancer, esp the blood.
The red on the paws you may have seen could have been from post mortum changes and rats sometimes take some time for rigor to set in depending on the cause of death.   I am sorry for your loss and I wish I could be more specific, but without a necropsy its just too hard to say what it was, although I do feel it could have been one of the few things I suggested above.