Could these be signs of pregnancy?

I am 17 years old and am freaking out, i can't be pregnant at this time in my life :( I have been on the pill for 3 weeks now and have been taking it every day but have not been very consistent on taking it at the same exact time every day. halfway through i took it a lot more consistently. I just had unprotected sex 3 days ago. I was supposed to get my period two days ago and all i have now is a light brown discharge that i have never gotten before and cramps. I am so worried that this is implantation bleeding which is an early sign of pregnancy. Please give me honest advice because i have no idea what to do and am so scared. I feel so stupid for being so careless :(