I got a pair of pants at good will wore them with my underwear on didn't notice that they were stained with dry white vaginal secretion and maybe a little blood. So freaked out I had in a good pair of underwear and I'm sure my body didn't come into contact with it but still freaked please help!
Although purchasing a pair of pants with secretions and blood may put a crimp in your style, it certainly does not expose you to HIV or other STIs. From your description, you wore a pair of underwear with a used pair of pants (always a wise plan!), and your skin had no contact with the spot in question. Even if it had, it is extremely unlikely to impossible to acquire a disease or infection from a dried spot in clothing, especially if the pants at some point had been washed and dried (as one would assume they would have been if sold at Goodwill).
When in doubt, regular testing and screening from a healthcare professional in your area will help you to resolve the anxiety and fear from this experience.
And if I were you, I'd go to Goodwill and get my money back!
I sincerely hope you feel better soon.