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.)