What Causes Back Pain After The Menstrual Cycle?

i got my earlier periods on 16 july,and this time on 14th august,now jst on 9th day of my periods i spotted brown bt very very lightly for 2 days and form that 9th day i have continuous light lower back pain ,and some periods like feeling in lower abdomen.we had intercourse on 12th aug and one on 18 or 19th aug,unprotected,can i be pregnant?