My lower left leg is swollen, (3cms bigger than my right) its sometimes tingly. I don't have dvt, I haven't hurt myself. What could be causing this?

Swollen leg. There are many reasons for unilateral swelling of a leg. I would start by repeating the venous duplex exam and if this is normal then an MRV of your vena cava and iliac veins would be the next step. If these tests are negative then I would suspicious of lymphedema and evaluate for this. Symptomatic treatment would be support hose, lymphedema massage and segmental compression pumps.
Many possibilities. DVT is the 1st cause to think about. To be sure its not present ultrasound may be needed. Other vein conditions (varicose vns, leaky valves, others), excess salt, heart, kidney, liver conditions, malnutrition, lymph problems, infections or parasites may play a role. Swelling itself is heavy and bothersome but not dangerous. Some of these causes can be dangerous, others not. Hope this helps.
Are you sure no dvt. It would be important to clarify that there is no dvt. Sometimes you can have a negative ultrasound on the first check and a second 1 is needed. Other things that can cause leg swelling maybe heart failure but you are a little young for that also infections in the leg such as cellulitis can cause this. In rare cases there can be something compressing vessels in pelvis causing this. Go see doctor.