Good Day , I Have Been Bleeding Since 15 Weeks

Good day , I have been bleeding since 15 weeks when they first told me I have an infection. They gave me antibiotics and I started to bleed ever since . I went to er and they told me I may b miscarrying but I did a ultrasound and a pelvic exam and they can't find anything wrong . They gave me gynotran and I was still bleeding so I went back to the ER and they said that I may have cervicitis and I may still b bleeding because I have a low lying placenta. Well for two days now I was passing blood clots but have no pain .. Besides
pain sometime but they say that back pain is normal and I am now 18 weeks .. Please tell me what is wrong