First it needs to be determined what kind of STD
you are suffering from.Sepmax DS is an antibiotic which is treating the infection.It matters whether you are getting the red spots on your neck after taking the drug or it was there before. If it has shown up after starting the drug,then it could be a drug reaction which may be seen in sulpha
group of drugs.In that case,stop the drug and contact your doctor for changing the drug. If the red spots are not connected to drug intake then it should not be an manifestation of the STD.Skin conditions needs to be ruled out first.About the like syphilis
etc can affect the cord an xray is important.