Implantation bleeding or period?

I have been on birth control for 3 years and never has it made me nauseous and never had breakthrough bleeding. I started my packet on Sunday October 12th and this past Saturday I went to take my pill and got very nauseous and threw up an hour later. I have had headaches, nausea, lower back pain, cramping, and heightened sense of smell. On Sunday I started having dark brown somewhat sticky discharge/blood? (its not red) enough to need a pantiliner but not a tampon. Its Tuesday and this is still going on. I have continued to take my birth control pills. I am still feeling nauseous, tired and have a headache. I feel like crap... Severely tired.. Does this sound like implantation bleeding/pregnancy symptoms or my period/breakthrough bleeding? Your answers are greatly appreciated!