When you carry more than 1 fetus in your womb, your bump will look bigger than those carrying singleton. And I believe what you're describing as lower abdomen cramp is caused by ligament round pain which happens and will continue to happen till the 3rd trimester. Your uterus is stretching to accomodate to your growing babies. Should not feel painful & it normally comes and goes. If it is, pls see a dr.
Could be ur uterus is expanding/stretching as u are having two babies growing inside. But I would also suggest u to call up ur clinic and ask ur gynae if the cramp is ok and if he/she wants to u go in for a check.
it is better to call your doctor .because you have twin pregnancy