2 hours after the encounter the tip of my penis started to tingle and I had to urinate a lot. At some point with in the weeks noticed some redness on the lower half of my urethra.

1 week post encounter I tested negative for chlamydia, gonorrhea, and trich.

3 weeks post exposure I again tested negative for chlamydia, gonorrhea, and trich. However, I did test positive for ureaplasma and yeast. I was given doxycycline 100 mg which i was prescribed for 10 days. I thought I lost some pills and got some more and long story short I took the doxy 100 mg for 13 and half days.

The redness has never went away nor does it appear a lession or ulcer has occurred.

I returned to the doctor and was prescribed an anti fungal that appears to be helping.

If I had been exposed to syphilis would have that amount of doxycycline been effective?

You describe a near zero risk sexual exposure -- i.e. little or no chance of any STD, including syphilis. (Also including HIV.)  No infection of any kind can start to cause symptoms 2 hours after exposure, and neither frequent urination nor urethral redness are STD symptoms. I have to believe both are mostly the result of anxiety lowering your threshold to experience or worry about these things. In any case, your negative test results are reliable. Ureaplasma is an entirely normal bacteria in the urinary tract and means nothing; you could not have acquired it from the exposure described.

You did not require treatment with doxycycline. Syphilis is also exceedingly unlikely from the exposure described, but if you had been expoed, it would have cured the syphilis before it got going.

At this point you need no further treatment or testing. If you remain concerned, keep working with your doctor. But I am entirely confident you have neither syphilis nor any other infection from the sexual exposure you are concerned about. 

I hope these comments are helpful. Let me know if anything isn't clear.

HHH, MD