Why does the stomach tighten up after a little movement post- tummy tuck?

.The swelling doesn't change much during the course of the day. Perceptual changes might based on what you expect of the abdominal muscles that are swollen. All of the symptoms will improve as swelling subsides. Full recovery may take several months

If you are doing something in particular tat is causing you tightness or pain then it is too much for you at this point in your recovery.

The ususal cause for your complaint of your tummy muscles tightenting up is due to spasm which resolves with time. 

When muscle work is done in tummy tuck the new tension across the tightened muscle can cause increased spasm when used such as in getting out of bed, sitting up or down ,or just walking. therefore you will feel a tightening. If this becomes painful you might benefit from a muscle relaxer. It resolves over 6 to 12 weeks.

The cause of the abdominal muscle tightening is usually muscle spasms. This improves typically over time and with rest. I encourage you to discuss your concerns with your surgeon. He/She is most familiar with the details of your surgery and how you've been progressing through recovery.