I am trying to post on the Mathematics Stack Exchange site, but the message I get is that you can only post once every 40 minutes.
The problem is I haven't posted anywhere, anything in any community in the last two days. Why is it happening?
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You said you were using the internet at the library. At that point, it's likely someone else already used this same library connection (with same IP address) to ask a question before you. From the Rate-limiting faq:
Users with < 125 rep on the current site, 40 minutes since their last question anywhere on the network (This applies to the user's IP address, not their account. If the user shares that IP with other users, they can be limited by the other user asking a question anywhere on the network; similarly, if the same user posts a question from a different IP address, they may not be limited.)