Not sure if I understand exactly how your particular flex spending health card works, but in general, this is my understanding of the rules (as set by the IRS):
1) Before a given calendar year starts, you elect to set aside a certain amount in your health care account. Let's say you decide to set aside $2000 into this account. Your employer will deduct $38.46 per week (or equally apportioned according to whatever your pay period is).
2) You pay your portion of medical bills throughout the year (co-pays, prescriptions, deductibles, etc.), and submit a claim reimbursement form along with receipts showing payment to get paid out of your health care account.
3) Let's say this is for calendar year 2004. You can only submit health care expense claims that have a service date in 2004. If you incur a medical expense on 12/31/2004, but don't pay it until 1/15/2005, you can still submit it. I believe that it must be received by January 31 of the following year (2005 in this example) to get reimbursed.
4) You can only get reimbursed up to the amount you decided to set aside ($2,000 in this example). If you end up having more out-of-pocket medical expenses than the elected amount for a given calendar year, you can't "increase" your election during the year. It's kind of a gamble - elect too little, and you miss out on pretax funds for paying medical bills. Elect too much, and you essentially give away that money.
5) If you don't submit claims for all the money in the health care account, you lose it / forfeit it.
6) You can submit claims up through your elected amount, even though your weekly contributions into the health care account have not accrued to the maximum. For example, you could submit a claim for the full amount, i.e. $2,000 on March 1st, and quit your job March 2nd. Even though your weekly contributions have not fully accrued to the maximum amount elected, you do not owe that money back. It's the risk the employer takes. However, if you have accrued lets say $400 into the account by March 2nd, and do not have any valid claims to get that money, you lose it. It's the risk the employee takes.
7) You can also submit claims for over the counter "health care" items: Band-Aids, Preparation-H, aspirin, etc.
This is my understanding, but I'd consult your benefits department for specifics of your plan.