Bleeding in second trimester

Samantha • 27 year old mommy, wife and kindergarten teacher.
Pretty much the entire second Tri, I'm 21 weeks now, I've encountered spotting at least once a week. It is only a small little droplet every time of bright red blood, and it's only on the toilet paper when I wipe... Never in the toilet or on my liner. My doctor has ran tests and can't seem to find a problem. I'm curious if anyone else has experienced this?