Here you have explain about a knot at your leg level from past 3 weeks and it has become bruised. Along with this there is a complain of ankle pain and swelling from past 3 days. But you have not inform where exactly the knot is present at ankle level or above ankle level its question. Now let me tell you if this knot is increasing in size and changing in colour then you need to visit your doctor for proper check up and treatment and you may need to go for excision surgery along with biopsy of the same for conformation of what it is. And after this if your ankle swelling is also over then good but if not then use crep bandage and keep doing ankle toe movements and do not strain your ankle with weight bearing. You will be suggested for certain medication also to reduce infection if it is. So Dont worry and follow the treatment program as per your doctor advice. Now if this knot is not increasing in size and its just a normal fluid sack then I suggest to use ice pack and hot pack frequently in day time on it. This will help to reduce it on its own. And keep taking precautionary care for ankle to reduce swelling and pain with use of crep bandage and elevating leg along with needed anti biotic and pain killer tablets. I am sure this will be helpful if you follow it properly step by step. Take care.