What Could The Lump On Penis Be Suggestive Of?

There is a lump on the lower side of the shaft of my penis. I first noticed it about a week ago and on two occasions I have been unable to maintain an erection since. I am worried this is Peyronie's disease, and want to know if there are any other possible causes (perhaps stress) of this lump. As far as I can tell my penis is not curved.