At 20 weeks, this kind of lower abdomen tightening and pelvic stiffness is usually your uterus practicing Braxton Hicks contractions, which are harmless and common, especially after lying still for a long scan. The pulsing feeling is often just your own pulse being more noticeable in the pelvis, not a problem with the baby.
Next Steps
Rest, stay well hydrated, and empty your bladder, since a full bladder can add to the tightness. If the stiffness keeps coming and going without settling, note how often it happens over the next few hours.
Health Tips
See your obstetrician the same day if the tightening becomes regular and painful, you notice bleeding or fluid leaking, or your baby's movements feel reduced.