Good Morning and welcome! I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
I am so sorry to hear that Flynn is not feeling well....
The most likely cause for this is arthritis that is making Flynn have a generalized pain and this causes him to not want to have a bowel movement because it hurts. When the stool remaons in teh colon too long, the hard stool will cause irritation to the very sensitive rectal mucosa causing small blood vessels to break and bleed. This will resolve once the stool is normal.
For now, give Flynn aspirin at 5 mg per pound every 12 hrs and give Flynn a daily supplement of Glucosamine/Chondroitin available at the pet store. This will add lubrication to Flynn's joints decreasing pain.
Do not allow Flynn to jump on anything or even climb stairs to allow the medication to work and stabilize a disc issue that could also cause this.
It is best to have your vet examine Flynn since his age could cause other metabolic issues that may require treatment.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please REPLY so I can clarify for you.
Sincerely, ***** *****