Normally I wouldn't worry, especially if her body condition is still good, and if she is still fairly young then she may have just hit the "1000 gram wall" a little late. And yes, one big bowel movement can make a difference in how much they weigh.
BUT...
- You bought a wild caught cheap imported baby blood python that never ate for you, had seizures, and then died last February a few months after you brought it home. Did you have a necropsy done on her?
- Your male red tail boa later popped up with mites.
- You've brought in two more boas in the past month, one of which was from a pet store.
- You live in an apartment so QT is going to be minimal simply due to lack of space; everything is under the same roof even if you have an enclosure in each room or on opposite sides of a large room.
If this girl's body condition deteriorates, she loses much more weight, or she has any marked behavior changes, I would schedule a vet visit and get bloodwork done.