Hi, im a guy, 16 and still a virgin untill 2 days ago and i was kinda taken advantage of by my sisters 19 year old friend, my sister went shopping whilst her friend stayed home, i was working late so i was assleep mid afternoon and i find her giving me a blow job and she sat on my face forcing me to lick her p***y/anus, at this point i was realy enjoying this and let her carry on, then she grabs my penis and puts it in her vagina using no protection what so ever and also i came inside her. the next day i comfronted my sister and told her everything but she doesnt believe me, and im worried about STI/STD, because i masterbated the next day and i noticed a bit of blood in my cum, is this a sign of Sti? is there anyway i can get tested without anyone knowing?

PLease i need advice fast...