I sympathize with what you're going through, but now is not the time to panic or worry, you need to "get your mind right". First off, looking up information on STD/STI's will obviously assist you in eliminating what you know you don't have, as it will also help you better understand each individual symptom for the closest relational disease/s you think you might have and their treatments.
Are you're suspicions of this affair leading to you contracting an STD/STI based on empirical evidence, or is it speculation born from mistrust and the thought of the unknown that has you so worried?.
If the affair has lead to you contracting an STD/STI, what kind of symptoms are you showing?. If you aren't showing any symptoms, and are still worried, then go get tested by your doctor.
If you end up not having an STI/STD, and have been overbearingly critical and/or disruptive towards your ex without evidence, then perhaps this behavior of yours is being expressed in other areas of your relationship, and is part of the reason why the affair occurred in the first place.
"am I being irrational?" - Buck up buddy, go tell that doctor that you want to get tested. Doctors are here to help people, not to judge you like a 10 year old would on the playground of a low class high school located somewhere in south America. (Only minor offense to South Americans)