It is completely impossible for a Chlamydial infection to return if it was treated successfully with Antibiotics.
What can happen however is that as we get older, our bodies can become more sensitive to certain bacterial strains & with men infection of the prostate can occur (BPH).
Relatively harmless bacteria could have made its way into the urethra & travelled down to the prostate, causing a flare-up of symptoms WHICH ARE NOT CHLAMYDIAL IN ORIGIN..
The other most common conditions are:
NSU/NGU which is both sexually & non-sexually transmitted (again through sensitivity to bacteria)
THRUSH/CANDIDIASIS - Yeast infection which can be commonly passed between partners - Symptoms include itching, redness burning, discomfort and sometimes discharge... THIS IS NOT AN STI.
I certainly wouldn't rush to any conclusions regarding any form of cheating until you know for certain what it is.
However the likelyhood is that it wont be an STI.
I absolutely agree with aldo. There are many things that could be causing your symptoms, which you don't describe, but before jumping to conclusions, wait for the results.
If it is chlamydia, it's not the same infection you had before.
Are they only testing you for chlamydia, or are they looking for other things, like a UTI, NGU, etc.?