From the Center for Medicare & Medicaid Services (CMS) - COBRA Q & A.
Q7: How long does COBRA last?
Assuming one pays all required premiums, COBRA coverage starts on the date of the qualifying event, and the length of the period of COBRA coverage will depend on the type of qualifying event which caused the qualified beneficiary to lose group health plan coverage.
For “covered employees,” the only qualifying event is termination of employment (whether the termination is voluntary or involuntary) including by retirement, or reduction of employment hours. In that case, COBRA lasts for eighteen months.
If the qualifying event is the death of the covered employee, divorce or legal separation of the covered employee from the covered employee’s spouse, or the covered employee becoming entitled to Medicare, COBRA for the spouse or dependent child lasts for 36 months.
Not sure about the particulars in your case. Follow mickeyd's advice and contact your company's HR benefits specialist.