Did convenia cause death?

Dear Dr. Marie,
My beloved pet has died just seven days after receiving a shot of Convenia. My query is should this newer medication be given to a 17 year old cat who is already in distress. She received the shot one week ago Monday, along with a script for mirtazapine to stimulate her appetite and began to become lame from that same evening. I, foolishly, thought her hind leg was sore from a blood draw the vet had done even though I told her I wouldn't be able to submit it due to the fact both my husband & I have been laid-off during the last few years and have no health coverage for ourselves and couldn't pay for it right now. I was told she was dehydrated and she received a subq treatment. She hadn't eaten in 3 days but was still drinking on her own. I keep finding articles where it is claimed that this drug causes anemia. This would explain the total loss of mobility which occured by this Sunday. It worsened each day and progressed to the front limbs. The vet claimed she had no incidence with this drug but I couldn't afford to bring her back in and she died in my arms early this past Monday morning. She tried to drink on her own all week but by Sunday I was using the syringe to give her fluids, which she did drink. I wish now that she had been given an oral antibiotic that I could have given to her each day. I was never asked. How could she have made such a drastic change within 6 hours upon returning home from the vet's visit. She looked zoned out at times. Until she vomited on the Friday morning she was her usual self; leaping across me from the sofa arm to an arm's length away like a furry little gazelle. What happened is all I think about now.