I have had a large selling/bruise on my leg next to the kneecap. I 've had it for at least 30 days , but it's not going away. So far, I've tried ice paks and several times a day elevating my leg , arnica cream,and massaging it . Nothing seems to be working. Very little pain, but instead of getting softer , its becoming harder . It's also getting larger. I suspect it is a result of bumpng it . I am 75 , a college graduate and live in Richland County,South Caroina. I have seen my general practitioner who recommended the things I tried so far, but because its getting worse, think I need to see a specialist. But, what speciality?

I'd start with an orthopedic doctor.

I agree, assuming you did get it after bumping your knee. But at 75, you also need to be considering vascular problems. The left leg is more likely to have them than the right, because the heart is on the left.
(This is just something to consider, not in any way a certainty.)