What Causes A Small Lump On The Penis?

I have a small lump which looks like a wart on the base of me penis. I had it lookd at 4/5 months ago as i thought it was genital warts but the doctor said it wasnt. Its now got a bit bigger about 4mm wide an 4mm across an stands the same in height. Its a bit sore wen u rub it but doesnt cause any discomfort. Its skin coloured maybe a little reder ?? I just hav the one little lump and hav had it for afew months