Is it normal for your stomach to get flabby after tummy tuck when it was initally flat?

Congratulations on having undergone the tummy tuck procedure. What you describe is commonly encountered after this procedure. Patients will like their “flatness” immediately after surgery; then, these same patients are disappointed for a period of time once swelling of the tissues occurs. In the longer term (this may take several months and even up to one year), the swelling dissipates and the skin of the abdominal wall "redrapes"  over the smaller underlying  musculoskeletal framework (where the abdominal wall muscles have been brought together again in the midline).
Sometimes, fluid accumulations (seroma or hematoma)  may cause the appearance of increased swelling; best to have your plastic surgeon evaluate you if you have noticed significant changes and/or have concerns. I hope this, and the attached link, helps. Best wishes for an outcome that you will be very pleased with long-term.

It is difficult to answer your question based on photos alone. Swelling can be normal intermittently for months after a tummy tuck, or it can be due to an issue such as fluid collection, relaxation of the rectus plication, constipation, etc. Your PS would be able to distinguish these causes with an examination and possibly tests.

It is possible that you are doing too much after surgery.  You need to let your body heal.  Pushing through the swelling will make things worse.  Some patients will have to rest for longer than others.  If you are patient, it will probably be fine.  It just make take 6 months or more.