On July 19th I Had Unprotected Sex And About 24

On July 19th I had unprotected sex and about 24 hours later I took Plan B one step. I experienced some cramping and brownish bleeding. On July 22nd I started taking birth control pills and the bleeding had stopped the next day. I had unprotected sex again July 25th. On Tuesday July 28th I began to have severe abdominal cramps and the dark brown bleeding returned but this time it was composed of mostly blood clots and chunks of tissue. This has also been accompanied with headaches, occasional back pain, and fatigue. This has now been going on for 2 weeks. I'm not sure when my next period was due because my period schedule is very irregular but it normally only last 5-7 days not for 2 weeks. Do you think this could be a sign of miscarriage or just an adverse effect?