It certainly is normal to have some pain and tight feeling 3 weeks after a Tummy Tuck. However pain associated with a feeling of pressure could be a sign of Seroma, a build up of fluid underneath the skin. Less common would be a hematoma, a collection of blood under the skin caused by bleeding.
If pain and pressure is associated with swelling, it may be more serious than the typical discomfort following a Tummy Tuck.
Always consult your surgeon and go in for an exam.