You might try feeding a little more frequently, unless she is the one setting the feeding pace by refusing a weekly meal. A medium rat is plenty big enough and bigger than many choose to feed. I certainly don't think you're feeding her so little that she couldn't poop.

Some of them just store it up and will let it go in one big monster movement. Has she shed for you yet? I'll bet you there may be a shed coming on fairly soon...and when she sheds, she'll leave you a big ol' stinky present wrapped in freshly shed snake skin.