The fact that Pita's "scent sacs," or anal sacs, filled up and had to be expressed was probably not because she was in heat. We don't completely understand at this time why anal sacs become overfilled and sometimes go on to develop into an impaction or abscess.
The anal sacs are located at approximately four and eight o'clock within the anus. These sacs act as reservoirs for the secretions produced by the anal glands. Some dogs, especially small breed dogs, do tend to have problems with their anal sacs accumulating secretions from these glands, becoming inflamed, impacted, and sometimes even forming an abscess.
However, the accumulation of fluids may be due to allergies or periodic soft stools or diarrhea, which Pita may have been experiencing when she entered her heat cycle. Now that she is spayed, you should have her sacs checked by your veterinarian in about three months to see if they are full. If she starts to scoot or lick the anal area, have her checked by your veterinarian sooner.
It may very well be that your dog's anal sacs will still have to be expressed every few months, depending on how quickly they fill up. Your veterinarian will be able to guide you on how often the sacs will need to be expressed.