It's impossible for me to diagnose your dog's condition over the internet. However, the symptoms you've described could be indicative of various medical problems, some of which can be life-threatening.
Here's why it's crucial to seek immediate veterinary care:
* Lethargy, lack of appetite, and vomiting: These can be signs of various conditions, including infections, poisoning, organ failure, and more.
* Bleeding from the rectum: This can be a sign of internal bleeding, intestinal problems, or even cancer.
* Inability to keep water down: This can lead to dehydration, which is dangerous and can exacerbate other health issues.
Do not attempt to self-treat your dog. Instead, contact your veterinarian immediately.
Here's what you should do:
1. Call your veterinarian immediately. Explain the symptoms in detail.
2. Follow your veterinarian's instructions carefully. They may advise you to bring your dog in for an emergency appointment.
3. If you cannot reach your veterinarian immediately, consider taking your dog to the nearest emergency animal hospital.
Remember, prompt veterinary care can make a crucial difference in your dog's recovery.