MySQL problem: User has already more than 'max_user_connections' active connections

[Mirrorball]

I created a test installation of Drupal, a CMS, on my server yesterday. Many hours later, I started getting MySQL errors from vBulletin (too many connections) in my inbox. Even though no one was accessing Drupal, the errors wouldn't stop until I deleted the MySQL user and created another. I changed all my scripts to use the new username and everything was working again. Then, a few hours later, the same problem happens and I change users again. At this point I deleted my test Drupal installation and no more errors occured.
What could be causing this? Drupal is not set up to use persistent connections. I had installed a couple of modules I created myself, including a vbulletin integration module. I suppose I could have made a mistake and a module could have an infinite loop, but it didn't look like it. And wouldn't the script time out eventually and release the connection link? My site wasn't very busy either, the first errors occured at 8am, not a high traffic time. And why did I have to create a new MySQL user to make the site work again? Does it make any sense? I'm very confused, I have no clue about what could be causing the problems. My site is at a shared server and last month was far busier than this one and I didn't have any problems.

vBulletin and Drupal were using the same MySQL user the first time it happened. Then I changed the user for Drupal, but the vBulletin module was still using the same user as vBulletin.

[hambil]

I've seen this problem on my server lately, too. If I find the cause I'll let you know.

[Mirrorball]

Now there are 21 users at my forum. This is very ordinary. But at 18:67, there were 271! It's IMPOSSIBLE. Previously the highest number of users was 101 and it was just after a major piece of news was released. Today there were more than 2.5 times that number of users, it's not normal. There must be something wrong somewhere and I don't have a clue what it is.

What is even weirder is that in the access logs, there were no more than 132 users per hour. vBulletin's 'Who is online' refreshes every 15 minutes, am I right? If there isn't any activity from a user in 15 minutes, the user isn't shown on 'Who is online.' How could there be 271 users online in 15 minutes if there were no more than 132 users in one hour?