What you describe is a mucocele, which occurs if you inadvertently bite your lower
lip and severe a minor salivary gland
in the
lip. They normally feel like tiny bumps on the inside of your lip- you can actually see them if you invert your lip (turn it inside out) and stretch it.
When you bite through one, the saliva pools in the lip. It can be clear or turn darker. It is nothing to worry about and goes away on its own after a few days, provided you don't keep biting it.