Yes, of course you are. If your company has a group medical insurance policy, it will cover all employees as long as they are employees when they incur that medical expense. Until you leave the company at 5pm on your last day, you are still an employee. Your claims can go straight to the group medical insurance provider and they will reimburse you as long as you can prove you paid the medical fees and you were still an employee at that time.

If your company does not use group medical insurance and covers the employee's medical expenses out of company expenses, you should still be covered. Read your employment contract again. See whether it says that your medical coverage ends upon the tendering of resignation, and not on termination of employment (ie. last day). All the employment contracts that I've seen so far, cut off on termination of employment.

Having said that, it really depends on how professional your company is. If they want to make it hard on you by citing reimbursement policies etc... that's just too bad for you. You have to fight or simply walk away & forget about it. Most big MNCs will cover all commitments to their employees up until the last day of work. In my experience, they will either issue you a manual cheque/petty cash on your last day or if they have to stick with their payment processes, their Accounts Payable will post you a cheque during their next batch cheque run.