I noticed a flat brown disc shaped form, with a center, like sunny side egg. 1/2 in" - not food, and also red blood and mucus in my stool. What is it?

Hard to say. If you have saved it take it into your doctor's office for pathology. Have your doctor examine the area. It may have been a polyp. Arrange to see a GI specialist to look at your colon. All the best.
Feces. I assume the brown substance was feces. A small amount of red blood and mucus may be due to an anal fissure or internal hemorrhoid or less likely ulcerative colitis,polyp or other growth. I recommend that your physician examine you and,if needed,appropriate tests will be ordered. I suspect a benign situation but you will have peace of mind after being checked.