Hello~If the underwear you ordered was not "used" and it probably was as Amazon does not sell "Used" products, then the stain was probably already there but you did not notice it. As to contracting an STD that way, very unlikely, unless it just happened an hour or so ago before your wearing them. I wouldn't worry, your health anxiety is just acting up, I can relate, I have it as well.
Hi, you cant contact any std from what you describe. Not possible at all.