Eastern Prickly Pear, Devil's Tongue, Low Prickly Pear (Opuntia humifusa)
Nice picture of the flower.
I believe what you have here is a different Opuntia other than O. monacantha.
It looks alot like a Opuntia humifusa flower, like in this link: http://davesgarden.com/guides/pf/showimage/258165/
Opuntia monacantha has a red stripe on the outer petals.
Like this one: http://davesgarden.com/guides/pf/showimage/32700/
Would you like your photo moved to the Opuntia humifusa page?