We found our marma-pants dead

I will keep this short
We had an awful smell in our house and thought it was a dead mouse as we have many traps all over hidden, none of my cats can get to though. We have an outdoor cat that we had spayed so we don't have more kittens. She didn't want out Fri night which was strange but OK as we thought she just was tired. She has been very quite for a few days but still herself. We think sat morning was when she was inside non the less Sunday afternoon we decided to search for her as we haven't heard from her and she's usually wanting out. She kept all the mice/rats at bay. She was an amazing cat.  When we couldn't find her I suddenly said... no no look harder. The smell was getting worse. So back down to basement with flashlights and there she was laying there. FoamING from the mouth. Blood out of her nose. And laying in poop and fluid. The first thing I thought of was she's been poisened. I say this cause we live between farms.  We have a sheep farm which she visits regular and across the hywy where we think she came from. We haven't seen the usual strays that stop by for a quick fight. Just very still and weird here. 
Does this sound like she may of eaten poisen that a rat ingested? Please help cause the guilt of not knowing if I could of saved her is killing me. I wish she would of shown us some clues. We could of taken her to our vets. I feel so scared. I have 3 indoor cats too. And a parot. What do I do? Anyone please... 

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