Trace amounts of anything aren't reason for worry. You might have been a little dehydrated, or it may have just been normal for you.
As GuitarRox and Pax said, you can get an STD from oral, but the chances are a lot lower than intercourse. Most experts don't think one act of oral warrants testing in the absence of symptoms, but if you're worried, you can always test.
You can transmit std's via oral sex. Here is information on this from the CDC. https://www.cdc.gov/std/healthcomm/stdfact-stdriskandoralsex.htm Herpes is one that can spread skin to skin, and if the other person had an infection in their mouth or throat, it can transfer. Gonorrhea, syphilis and clamydia are possibilities. The risk is not huge, but still a possibility. I'm not sure what type of otc test you took but it doesn't sound like an std screening test. I would say that in the absence of symptoms, I'd not be overly worried, however, std's can be symptomless. Here's some information on testing. https://www.plannedparenthood.org/learn/stds-hiv-safer-sex/get-tested
It's highly you got anything from oral sex, and who knows whether an OTC test is accurate or not. Nitrates and nitrites are in lots of things, including processed meats and a nutrient you need to live. Nobody here is going to tell you if your test indicated anything; if you're that concerned get a real test from a doctor and a lab. But as you seem to be developing an anxiety problem, you might want to consider help for that.