Aw, totally understandable to worry, lalo na first baby mo! Mild cramping at lower back pain at 18 weeks can be normal, maraming kaibigan ko na nag-experience din nito, usually ligament stretching lang or Braxton Hicks starting early, pero since your OB dismissed it without really checking, I'd honestly suggest getting a second opinion or asking specifically about it sa next visit, kasi peace of mind mo importante din. Some doctors can be dismissive talaga, especially if di sila nakikita ang obvious signs, but that doesn't mean your symptoms aren't real! Maybe try noting down when it happens para ma-report mo properly, if it gets worse, regular, or may ibang signs like spotting, go to the ER na lang to be safe. You know your body best, mama!