Yep, I had that happen in my first pregnancy. I lost my plug at 19 weeks, and then even dialated to 2-3cm!

But I was put on some meds to help with the contractions and bedrest and I even made it another 5 1/2 weeks being that dialated! So just try to take it as easy as you can and there is a good chance you can make it to term or at least to where baby would be okay if he had to come, ya know?
That is so scary though! I am sorry you are going through it but it is very encouraging that your fFN test was negative. I have also heard of the plug regenerating so I think there is a lot of hope

Hang in there mama!!