I've had this hard lump on the shaft on my penis for maybe a week now. the lump is hard to see but you can feel it, this lump has formed 2 days after masturbation and i've already tested for sti (negative). the lump has not got any larger or smaller?

This could be a lymphocele that can occur after intercourse and,I assume, after masturbation. They usually resolve on their own & I'd give it a several more days before consulting a urologist for evaluation and advice.

If you had no sexual exposure then why the STD (sexually transmitted disease) test? The lump has to be examined for diagnosis as it could be one of many things depending on location etc, eg clot in the top dorsal) vein, trauma from masturbation etc. Pl see a Urologist and refrain from masturbation till you see one -to avoid any further aggravation to the problem. good luck