Please help me, I'm really scared!?

I'm 20 yrs old and have been on the pill since January (since being with my boyfriend). I've been taking the pills back to back and last week started noticing brown discharge with some bleeding. This has gone on for a week and today I've started feeling very tired with a headache. I know this is a sign of pregnancy. I went to the doctors today and she said she thinks it will most probably be because of taking the packets back to back. She made me take an std test. I'm really scared that I might be pregnant. I forgot to take one pill last packet but took it as soon as I realised (14 hours later).

Please someone give me some advice or assurance, don't know what I'd do if I became pregnant.