COBRA is very specific. You must pay your premium by the thirtieth day after the premium is due. If you do not, your COBRA rights end. Some employers will give you some leeway, particularly if there is a good reason for the delay, but most don't.
They may very well have canceled due to the loss based on your med. condition. However, if you paid late, you gave them the open door to cancel. You will have a very difficult time claiming unlawful discrimination, etc. based on late payments. If they accepted late payments before, it doesn't mean they have to in the future.