Dog had lymphoma, questions about diagnosis

June, 2011, I took my then 9yr 6month old lab for a checkup.  Everything was good.  Mid July I noticed she started to cough, a hacking cough.  Heart worm test negative, and took her back to the vet's July 22,2011. I dropped her off in the morning because I called on a friday and they had no openings, but allowed me to drop her off and they would see her in between other patients.  Afternoon, the vet calls me and states that he didn't think it was CHF, and he could "feel all of her lymph nodes" and that it could be cancer or even an allergy  He asked to do blood work, which I said yes.  Later he called back and states the blood work was normal.  I pick her up and receive cough suppressant medication and amoxicillin for the enlarged lymph nodes.  August 22, 2011, I take her back to the vets.  She now has numerous sores on her body and continues to cough.  We see the other vet, and she does an xray and checks the sores.  xray shows enlarged lymph nodes in chest and sores show t-lymphocytes.  She is certain it's lymphoma. We forgo, the biopsy and choose to try to make her comfortable and put her on prednisone and keflex for the sores. The next week I call the vet back because she has rapid breathing and heavy panting, vet tells me it's due to the prednisone, and they would like her to be on it for at least one more week.  Sept 5, we take her to the emergency vet, due to the rapid breathing, but also, she has pitting edema in her front paws.  We make the difficult decision to end her suffering that day.  The emergency vet stated the prednisone wasn't working.  My question....why was her cbc normal just one month before? Why didn't the vet recommend doing an xray in july?  I'm grieving and now I'm mad, because I feel like something was missed or worse...wasn't done.  Any advice would be greatly appreciated.