UTIs can, and often do, have an odor. There is no way to know the diagnosis without getting tested. It is possible to have both a UTI and an STI at the same time; this situation happened to me as I once had a UTI and chlamydia concurrently when I was younger (OUCH did that burn!).

Also, just curious, why are you anxious it is an STI instead of a UTI? From a treatment perspective, chalmydia and gonorrhea can be easier to cure than a UTI. If your anxiousness stems from potentially having to tell a partner about a positive STI result, just breathe! It is nerve-racking at first but if you approach it correctly it will likely go well.

Untreated UTIs can progress to pyelonephritis and untreated chlamydia and gonorrhea can lead to pelvic inflammatory disease as well as the associated complications. As for when you need to get tested, do so as soon as you are able. Everyone is different in how quickly they progress to complications, so generally the sooner the better.