"Why didn't you compete in the annual sports event? It was because you knew that you'd lose anyway!"
Or, "How did he run that fast? he must've taken steroids"
It falls under unwarranted assumption, but what is it specifically?
According to Douglas Walton a fallacy is described as the following: (page 270-1)
...a fallacy is a sequence of argumentation used in a context of dialogue (of which there may be many types) as a tactic of deception to trick a speech partner in an exchange, or as an underlying systematic, and serious type of error reasoning. Note that a fallacy, according to this conception, is not just any error, weak argument, or violation of a rule of dialogue, but a particularly serious and systematic type of error or sophistical tactic of an identifiable kind, used in argumentation to obstruct a goal of dialogue, or interfere with its realization.
Answering one's own question does not seem to involve a systematic type of error or a sophistical tactic. It is unlikely that it would be a fallacy by itself, but it may involve unkindness as the example suggesting that one did not participate in a sports event was because one knew one would lose.
Bo Bennett describes what he calls a pseudo-logical fallacy that may be related to answering one's own question:
Argument by Rhetorical Question: Setting up questions in such a way to get the answers you want. This is a name for an argumentation strategy covered by both the loaded question and leading question fallacies.
Bennett, B. "Pseudo-Logical Fallacies" https://www.logicallyfallacious.com/tools/lp/Bo/LogicalFallacies/6/Pseudo-Logical-Fallacies
Walton, D. (2010). Arguments from ignorance. Penn State Press.