Important questions

Okay so I had sex about 3-4 weeks ago and my period was right on time according to this app, but usually comes the 10/11. I got it the 12, and it's already really slowed down and never really even got too heavy! It's red, but the first day was super dark. Am I having implantation bleeding or?? I'm only 17, my parents will literally kill me and disown me. What can I do if it is implantation bleeding??