Soft cervix at 21 weeks

Anyone experience this? It was discovered I had some debris in my amniotic fluid so the MFM checked my cervix and said it’s soft so he wants me to start prometrium again. I left without thinking too much, otherwise I would have asked him these questions myself, but now that I’m looking into it, it’s freaking me out.

They didn’t mention a weak cervix when measuring my cervical length. He only said it was soft after examining it before the end of the appointment.