When you're laying in bed, most likely with your boo, & your arm is either under or around them & it's awkward or going numb. &/ or your boo could be lying on their own arm strangely so that you can be super close. You'd say "babe can we move, I've got weird arm".

When you're laying bed with someone all intertwined and one of you has weird arm. Lying on your arm in an uncomfortable position.