Host shut us down due to our usage

[pastapiggy]

Hi, I'm hoping somebody can help me here as I am pretty desperate.

I run a small community of about 100 active members (last 30 days), 35 members per day, 5-15 online at most times. Nearly 200,000 posts. Hopefully that gives you guys some idea.

I have had this forum running 3.6 since November 2010 without any issues. Today my host shut us down because we were using some obscene amount of resources (we are on shared hosting). They said they only just realized this now, but it has been happening for at least a month, they say.

VPS hosting is out of my price range, so my only option is to get this fixed. They have put the site back online to allow me to minimize our usage.

Apparently, most of it was coming from showthread.php (?). I'm doing my best to do all the suggested optimizations and stuff. My forum is heavily modded however, and I've never really paid much attention to how much resources the mods are all taking up. I've disabled the majority of my products in the ACP for now. However, my members are very attached to the features, and I'd like to keep as many as I can.

So everything said, I have a few questions.
How can I monitor my usage?
Would this be due to certain mods and if so, how do I find the culprits?
What else could be the problem?
What would you guys do in my shoes to solve this issue?

I'm sorry to be such a noob, but I've really picked up everything about running a forum as I've gone along and this is the first time I've run into this problem. My forum is mainly a little for-fun project of mine.

If anyone could help me here, I'll love you forever. If you need any extra info, just ask.