Typically, a clear discharge doesn't indicate an infection, but since it's new, it's good you're getting it checked out.
Pre-cum means pre-ejaculatory fluid, so that would normally be expelled with ejaculation. Are your ejaculations normal?
Have you actually been examined by a doctor? Have they tested your urine for any white blood cells or bacteria, or did you just have STD testing?
This may be nothing, but it's important that you rule everything out.
Im just gonna copy what AJ said to be honest... You had the oral a long time ago... likelyhood is that any symptoms would have presented along time ago... on the rare chance you did get something. As already said oral inherently carries a low risk for all STIs and it is important to remember that.
NGU or NSU can be caused by sex but also by normal bacteria and irritation. EG changing soaps.
utheruitis by itself is NOT an STI. It just means that the urethra is inflamed. It is often caused by chlamydia but in lieu of a positive result obviously it is not. It can also be caused by...
New washing power
CONSTANT CHECKING AND TOUCHING
and MANY other things