Error in Piwik (tracker): Too many connections (no replies)

Hi,

I started seeing this since recently:

[25-Apr-2015 04:06:09 UTC] PHP Warning:  mysqli_real_connect() [<a href='function.mysqli-real-connect'>function.mysqli-real-connect</a>]: (08004/1040): Too many connections in /serverpath/piwik/core/Tracker/Db/Mysqli.php on line 83
[25-Apr-2015 04:06:09 UTC] Error in Piwik (tracker): Too many connections

It happens rarely on a site that does not get much traffic, around 150-250 visits a day.

To me, it looks like it's only under certain conditions, and I wonder what could cause such an error.

What I could recall is that (long ago) I would get this error for vBulletin forum I have on this site:

mysqli_real_connect() [<a href='function.mysqli-real-connect'>function.mysqli-real-connect</a>]: (42000/1203): User some_user already has more than 'max_user_connections' active connections /serverpath/vbulletin/includes/class_core.php on line 1356

This error would also happen rarely just as I'm seeing Piwik errors now.

It's obviously something about MySQL, but I have no idea what and why.

I thought that someone could give a hint that could help.

Thanks