What Causes Painful Bruised Lump On Leg?

Good Day I have a big bruised lump on my leg which just appeared all of a sudden last night. It started with a stinging feeling and when I felt my leg I just felt this huge swollen lump that is blue. I did not bump or hurt my leg in anyway. This morning it has gone down a bit still but is still bruised and swollen and it is painful. It now seems like the pain is travelling up my leg. Please can you tell me what it can be and must I see a doc.