Thumbnail for Excerpts – out of memory issue

I do hope someone can help. I just received this message from my web host:

‘It has come to our attention that your account is using extremely large amounts of CPU power executing PHP scripts. I notice that your site is running wordpress – did you recently enable a new module or setting that may be causing the additional load on the server?’

I haven’t personally changed anything on the site over the last week or so. The only change I can see is that my client has added a new post in one of the categories and for some reason the category page won’t load (out of memory). I have the plugin, Thumbnail for Excerpts, and when I exclude this particular category in the admin, the page loads fine.

1) The other category seems to be working with thumbnails enabled so why would this not work for the first category? I’ve resized the 4 images that appear in the one and only post which is in the problematic category but this didn’t fix it.

2) Please could someone confirm that the Out Of Memory issue would indeed be the cause of the high amount of resources on the server or should I be looking in another direction?