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

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.