My Last Period Was On 7Th July. I Had Unprotected

My last period was on 7Th july. I had unprotected sex with my husband on 11th day of period. Had idoze72 next day. I see ight brown discharge and back ache on 13th day. Why is it happening. When can I expect my next period. My cycle is 26_27days regular. Am worried. And when they say that this pill causes irregular period for how many cycles will it last. This is the first time i had this pill. Please help