I know you are angry, but please don't take it out on your parents. It's not like this decision is an easy one for them to make. They couldn't win either way because you also would've been angry if they didn't tell you right away.

When my 11 year old Golden was diagnosed with cancer earlier this year (also on a routine checkup) I made the decision to put her down while she still "looked fine." The minute she started acting different (about 2 weeks later) I took her back in. I never wanted her to suffer and I never wanted her to feel like she was a disappointment or a burden. I wanted her final days to be happy. I wanted her to walk into the vet (a place she actually loved going) on her own without having to be dragged there in pain. I'm sure this is completely crazy to some people, but in my mind it was the best for her and I don't regret my decision for one minute. I didn't want to be selfish and keep her around for another month just to keep me company.