I've had tenderness - like a bruise - at the bottom on my ribs for about a year now. When I sit down it mostly hurts. I also have asthma.

Unlikely asthma. Asthma symptoms are cough, wheeze, shortness of breath, chest tightness and difficulty breathing. Rib tenderness would not be a symptom of asthma. Possibilities to explain rib pain are fracture from trauma or other muscle-skeletal issue involving bone or small joints where ribs connect. The pain with a certain motion points to the muscle or bone and not the lungs.