Why do I vomit blood after chemotherapy?

YOU SHOULD NOT! You need to talk to your oncologist right away and work this out. Vomiting blood can be quite serious and the medical team needs to know about this immediately.
Many reasons. Vomiting blood can occur for several reasons: 1. Continuous vomiting can lead to tears in the esophagus. 2. Low platelets from chemotherapy can cause spontaneous bleeding. 3. Cancer can be near stomach or esophagus and bleed. If it is heavy bleeding and happens a lot, you need to tell your doctor and evaluate the possibilities.