When is my employer permitted to terminate my health insurance if I am still an employee?

I am employed as a mortgage loan officer working solely on commission. I enrolled in my companies health insurance in August 2012. I was told that since I was new and on commission, I could still enroll but my portion of the monthly premium ($135/month) would accrue until I earned commissions and then it would be deducted. I never actually closed any loans through September of 2013 but I am still an active employee with the company. They notified me on September 3, 2013 that if I didn't pay the $1800 that had accrued over the past 14 months to cover my portion of the premium that my coverage would be terminated on September 30. As of September 30, they informed me that my coverage was terminated. I was not given any options for COBRA or anything else but I am still an active employee. Are they permitted to terminate my benefits while I am still employed? The only option I was given was to pay what was owed and then I could re-enroll in January 2014 when they have Open Enrollment.