3 weeks ago I was elbowed hard in my lower abdomen. Instead of bruising, it felt like a knot or something. I still have it, what could it be?

Hematoma? It could be a hematoma or pooling of blood under your skin. Please see your doc. Peace and good health.
Possible hematoma. You may have a collection of blood under the skin which is known as a hematoma. You could also have a contusion. Either way i would apply warm soaks twice a day and see if it gets better if not see your doctor.