Can health insurance decide not to pay?

In 2009 I was bein canceled off and went to obgyn, found out I had infection and was pregnant. Meanwhile I was still covered for 3 months and the insurance co. Paid visit and labs and then second visit I had medicine and ultrasound that they covered. I filled medicine and it denied it. Doctor called said insurance canceled payments and I got bills in mail for three visits, labs, ultrasound. They stopped payment because I was pregnant. Now other ppl have same insurance don't live at home and are pregnant are older then what minimal age requirement is, aren't in school etc. And they are covered. I was 18 living with mom who had insurance and was attending school online. It was sept 5th and coverage ended 90 days after 7/27 so I was covered.