What is the risk of sending information from a secured page over an unsecured internet connection?

I paid a bill by credit card on a supposedly secure site, but a pop up informs me that the info will be sent over an easily accessible unsecure connection. Is that information only exposed during the transaction, or does it linger forever for the world to see? How long shoud I "sweat it out?" Should I immediately cancel my card?What is the risk of sending information from a secured page over an unsecured internet connection?Don't gamble I would rather be safe than sorry