Hi mama, mucus plug discharge is usually a sign na malapit na, pero hindi always instant labor talaga, some of us moms actually waited a few more weeks pa after seeing it! Since you're not seeing any bleeding and baby is still active, that's good, but since you're experiencing lower back pain too, please check with your OB agad para ma-assess kung gaano ka-dilated ka na or if there are any other changes, better safe than sorry, especially with your due date pa lang in October. Hang in there, you're almost there na!