the causes of bleeding in second trimester pregnancy
mainly related to placenta,
it could be placenta previa
, if that is present your doctors might have told you about that.
the other things are local trauma to geniatal area, presence of cervical polyps, post coital bleeding etc.
as blood supply to the local genital organs increases in pregnancy, mild friction may also lead to spotting.
normally this type of bleeding will subside by itself,
anyhow if this is becoming continuous and excess you better visit doctor once and get examined.