Wa'alaikum assalam! Memang ada possibility tu, a few of my mum friends kena gitu, especially dengan kembar, sebab hCG levels lambat naik initially atau spread differently dalam body. Yang paling reliable is blood test, honestly, sebab dia detect hCG levels lebih sensitive daripada urine test je. Gejala-gejala yang you mention (cramps, backache, headache) memang boleh jadi sign of pregnancy, so I'd suggest you go see your doctor untuk blood test atau ultrasound, confirm je apa sebenarnya happening, sebab kalau pregnant especially kembar, you want to know asap for proper monitoring kan. Jangan tunggu lama, it's worth checking out just to be sure!