Kneeling on pavement places a lot of pressure on the sensitive skin and bones of the knee. If you were leaning forward and back as you pulled the weeds, that also exposed the area to minor trauma as you rocked. Bony areas like knees and elbows are prone to bruising because of their thin layer of fat underneath the skin. When you kneeled on the pavement, the tiny blood vessels just under your skin, called capillaries, became damaged and leaked blood. Some people have more fragile capillaries than others.
Women are prone to bruise from minor injuries, especially on the thighs and upper arms.
For most people, this condition isn't serious and doesn't need attention. The bruises may last for days or weeks and change color. There isn't anything you need to do. In the future, to avoid the unsightly bruises along with the stiffness and tenderness you are feeling, consider buying a small pad to kneel on or some kneepads.