It is highly unlikely that you'd get syphilis - or anything else - from protected sex and protected oral. What makes you think you got syphilis?
Is this the March encounter you are worried about? If so, you wouldn't have any neuro symptoms of syphilis. Those don't happen for literal decades of untreated syphilis.
Right now, you'd just have a sore on your penis, and maybe a rash.
The doxy wouldn't affect your testing, and assuming it's been 6 weeks since the encounter when you tested, your results are conclusively negative.