Posts appear then compress

Hi, just upgraded wordpress plugins and various other plugins, and my site appearance changed. Already deactivated each plugin one-by-one, didn’t help.

My site: http://www.writingwithchopsticks.com

Problem I’m having is when I go to one of my category pages, i.e. Mom’s 52, the page, for one second, shows my posts as thumbnails, as it should, but then, all of the thumbnails compress into one, and all I can see is one of the thumbnails (earliest post), with my right sidebar floated to the left.

The way the page initially appears, for that brief second, is how it should always appear.

I think what happened is when I first set up the site a year ago, I mistakenly made changes to the parent code (bad, I now know) instead of the custom css, so when I (finally) upgraded, it got rid of those codes.

Please help me with the code I’m to enter into my custom css to get the thumbnails evenly spaced again. THANK YOU in advance!