What Causes Lower Back Pain And Numbness In Legs During Pregnancy?

Im 33 weeks along in my 3rd pregnancy and I have had some good back pains for the last 4-5 day s were my legs feel numb at times or they feel really weak the pains are not to the point where i cant handle it.... i also noticed a brownish pinkish mucas last night when i went to the bathroom .... my doctors sending me for an ultrasound which is on tuesday because he cant tell if baby has dropped or is larger than should be (belly is measuring bigger than should be) just wondering what i should do