We can't know what this lump is from your description. Please go see your doctor and get it looked at and treated.
Wounds on the lower legs can turn into ulcers and worse, particularly if you are diabetic.
Please see your doctor and come back let us know how you are going.
It could be a blood clot. If a vein leaks it could create an ulcer that is tricky to heal. Get it checked NOW.
I am not a medical professional. I know of this from personal experience. It could be a cyst and the worse thing that you can do is pop it. If you do that you can give yourself an infection. I would state that you have to go to the doctor so that they can take a look at it and tell you their action plan. I do have a motto. If it hurts that is not normal...