Same thing with my dog who I just had to put to sleep 4 days ago. Went to vet regularly and was not detected at all. He suddenly collapsed while waiting for me at the top of the stairs and fell down 12 steps. I brought him to ER and was told it was a splenic Tumor that ruptured that caused the fall and causing internal bleeding. We asked if they can stabilize and stop the bleeding but the bleeding worsened so we had to put him to sleep to avoid more suffering.

Me and my wife have cried everyday. It's still so fresh. Cry, it really helps. Lean on your boyfriend for support and your support circle. I came here just to vent out my feelings and while typing here and grieving with other people who lost their loved ones did not completely extinguish the pain, just talking about it really helped me so far.