The usage limit depends on what you are doing with MySQL databases and queries. Having just one database does not mean that you are guaranteed to not get suspended. For all we know, you could have sent a thousand queries to that database in the course of minutes (although I doubt it). And you will not get suspended for just “no reason,” trust me. Admin can tell you more about this, so you can await his reply for more information, as us “normal” users have no access to your account (obviously for the better).
I am not really experienced in wordpress, but I do know that it does use databasing. It could be that it was accessing a database, but I would not know. Admin (or others here) will likely assist you with more useful information.
Admin won’t really have much more to say then what has already been said. MySQL suspensions are rare now that the servers have been updated. The MySQL suspension works a bit differently than the rest, suspending the user(s) with the most usage if a server gets overloaded. If the server was overloaded by all the websites that use it, you may have just been the unlucky one that generated most of the hits.
And like @Thewebuser22 states, WordPress is pretty database-dependent. So if you have a lot of page loads, your MySQL usage is going to shoot up.
The servers don’t really keep detailed logs on this stuff, so there really is not much else to say. The suspension will be over in about 20 hours, so just take a break from developing for a day. If you really cannot wait, premium hosting is an option.