Is it normal to have bad cramps at 18 weeks pregnant?

My sister is 18 weeks pregnant and last night was having bad cramps around her uterus and abdominal area. She just had an ultrasound two weeks ago and they said everything looked great with baby so far. She’s worried so I thought I would ask here. She has had no bleeding or anything else and she has not had any cramps today. Anyone else been through this? Is this a reason for her to be concerned?