Light bleeding and back pain for 4 days straight?

Julia
Hey girls! I may be overreacting or just overly worried but whenever something's the slightest bit different with my body I want to know why! My period this month ended on the 11th, and on the 17th I started bleeding again. It was first very light, like spotting, but it became a little heavier to where I had to wear a pad. It's the 21st now and I'm still lightly bleeding, and I've had some back pain for the last 2 days. (My back cramps while I'm on my period rather than my abdomen, though the pain wasn't as severe.)
I've just never spotted before so I'm not sure if this is normal. I'm sexually active so I was worried it could be a miscarriage... as well I took plan b around the 15th? So I thought it could be from that. Either way, I appreciate all answers! Thank you xoxo