Honey, I think this might be a slighly bigger issue then listeria for you. I completely understand because I was the same way. I worried and stressed non stop...could not relax about anything. This is normal to a certain extent, but when the fears begin to become a little bit irrational, or are preventing you from enjoying your pregnancy, it's time to seek help.
Keep in mind that worry is a normal part of pregnancy. Add to that the crazy insane things our hormones are doing, and for some of us it can become a bit out of control. My biggest regret is that I worried so much, I truly never had the opportunity to enjoy. I am only saying this to you because you say you worry about everything, and to be honest, pizza slices from under a heat lamp are not the typical worry things. I do not in any way shape or form intend to be disrespectful, so I hope it doesn't come across that way.
If you are worrying as much as I am getting the impression you are, talk to your Dr. I wish I had.
Congrats on your pregnancy, and I wish you a healthy and happy remaining 23 weeks!!
Listeria is usually from raw or under cooked food that has been contaminated. Pizza is usually cooked well enough that listeria isn't an issue.
To cause illness, the pizza would need to be in the "temperature danger zone" (between 41°F and 140°F) for an hour or more (the USDA recommends refrigerating leftover within 2 hours) and it if it did cause illness would likely be from salmonella (E.Coli is also possible but less likely). Most food places don't leave food out for more than an hour.