My reputation is less than 125. I did not ask a question 20 minutes ago. But the system reminds me I can only post questions every 20 minutes. What is the problem?
There are at least 27 other users who've accessed the site from the same IP address as you. And as Yannis mentioned, these rate-limits are enforced at the IP level (since the intention is to prevent Tupac-style abuse, limiting it by account alone wouldn't help).
You should be able to wait a bit and then post another question. Once you have over 100 reputation points, the limits are less strict.