Aiyooo twins mummy, congrats! Honestly ligament pain can come earlier with twins because your body is stretching faster, a few of us with multiples felt it from second trimester onwards, so 15 weeks isn't unusual at all. The one-sided pain is quite common too, sometimes baby position or how you're carrying makes one side work harder than the other, but since it's your first time with twins better check with your doctor just to be safe they rule out anything else, okay?