In order to prevent abuse — but also, I imagine, as a nudge — a large number of actions on Stack Exchange sites are "rate limited". The complete list of these rate limits can be found on Meta Stack Exchange:
For asking questions, there are a number of these.
- Users below 125 rep cannot ask more than one question every 20 minutes.
- Users are limited to 6 questions per day.
- Users are limited to 30 questions per month.
- Users below 10K rep will trigger a CAPTCHA if they ask more than once per 60 seconds.
Users above 10K rep will trigger a CAPTCHA if they ask more than once per 30 seconds.
Additionally, some users may more severely limited if their previous posts have been poorly received (e.g., downvoted and closed).
The rationale behind these is probably two-fold:
- we don't want the occasional spammer to flood the front page with "questions"; and
- as part of the ideal of "optimising for pearls", we want users to be asking good, well thought out, researched questions; asking too many questions too quickly can be an indication that the user is not putting enough effort into asking their questions. (By making the ability to ask questions a limited resource it is hoped that user will be more conscientious in its use.)
(For you in particular, I can't see why you would have recently hit any of the asking rate limits.)