I generally try to clean myself thoroughly after a bowel movement. However, on motzoi Shabbos I had a feeling that I was dirty over there, and when I went to the bathroom to check, I realized that I was significantly dirty over there and my underwear was stained from excrement. I do not remember when my last bowel movement was, nor do I remember how well I cleaned myself afterwards. I assume that I cleaned myself properly. I do not know how this happened. Do I need to repeat krias Shema with its brachos and Maariv? What about davening tashlumin for Mincha and repeating Havdalah?
No. You don’t have to say anything over. The reason is that if a person finds excrement after they already davened, and it was in a place that the person should not have suspected it to be there, then the person does not have to daven again. In your situation, assuming that you are a healthy person, and these things don’t happen to you, therefore once you cleaned yourself properly after using the bathroom, we assume that you are clean and that you will stay clean until you use the bathroom again. Therefore, you do not have to repeat anything.
Shulchan Aruch O:CH 76-8, M:B 76-32,33.