There are actually quite a few threads on here that debate whether it is "ok" to refreeze, and the camps seem fairly evenly split. I refreeze once and have never had an issue. Note: This does NOT apply to prey left overnight - those get tossed if they aren't eaten.

As per your original post - Get a scale and monitor your snake's weight. That and body condition are the only indicators that can tell you whether you should be worried about a fast. Also, depending on your definition of a "medium" rat (standard is ~80-120g), you could actually be feeding larger prey than is necessary, which can lead to refusals also. My 3000g girl still only gets one small (50-80g) rat per week and she's a beast!