What Causes A Brown Vaginal Discharge With Strings Of Mucus In It?

Hello. I'm 21 years old, with irregular periods. I just started the nuvaring in attempts to regulate my periods so I can attempt to have another child later on. The week I took the nuvaring out, I didn't get my period. However I am now spotting with light pinkish and a little bit of brown blood along with long strings of mucus that have strips of blood through the mucus. What could be the cause of this? I have taken a pregnancy test (though I'm not sure if it would be to early for actual results) and it came up negative.

Also a little medical history about myself: I've had two premature children due to uterine infections in the past. My son was born 27 weeks gestation and my daughter (who is no longer with us) was born at 28 weeks gestation. I'm not sure if perhaps this could have any factor into this current situation but I do have a history now with uterine infections.