Port Change after setting microcontroller asleep

I can't help myself and didn't find any information on this problem on the internet so far.
I'm working with the SleepyDog library (Adafruit) with Arduino on my Windows computer.
After setting my microcontroller asleeep and waking it up againg I can still send new commands to the microcontroller but the serial monitor doesn't show me any new outputs, which it should.
After only setting my microcontroller asleep and trying to compile another code, the Port changed (although I didn't change any of the settings) and the Arduino Software couldn't find my microcontroller.
So I had to manually change the Port back to its original.

My conclusion is that after waking up again another port is used wherefor the serial monitor is not connected to my deivce any more.

Does anybody know how to solve this problem?

I also tried to set a fix port in the device manager on my computer, but then I can't compile it any more at all because it says the port is already in usage.

Please show your code. Just in case you haven't read it, see point 7 in How to use this forum - please read on how to post code.

Which Arduino are you using?