Tightness that stays in one spot near your pelvis or the area just under your navel, especially when it eases with rest or drinking water, is usually round ligament stretching or normal mild uterine tightening in pregnancy, not a cause for alarm on its own. The little vibration and mild cramps you describe fit the same pattern.
Next Steps
Keep track of how often it happens and whether rest and fluids keep settling it, and mention this pattern at your next antenatal visit so your obstetrician can check your cervix and baby's position if needed.
Health Tips
See your obstetrician the same day if the tightening becomes regular and frequent, doesn't ease with rest, or comes with bleeding, fluid leakage or reduced baby movements.