[In order to prevent abuse](http://blog.codinghorror.com/rate-limiting-and-velocity-checking) — but also, I imagine, as a [nudge](https://en.wikipedia.org/wiki/Nudge_theory) — 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:

* [The Complete Rate-Limiting Guide](https://meta.stackexchange.com/q/164899)

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:

1. we don't want the occasional spammer to flood the front page with "questions"; and
2. 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.)