grains are caused by:
* over-exposure and underexposure.
overexposure example:
(you get this effect, when you adjust an almost dense "black" negative to show details, the effect is unsharp + grainy)
Underexposure example:
(black black and at extreme example, a lot of screen artifacts)
* Film Emulsion / Developer Combination
Expired film example, tmax3200 expired since 2002:
I do not have a tri-x and rodinal example, the grains are huge.
Here is an example of fomapan developed in rodinal stand for a whole hour at 30c.
* Overdevelopment and overexposed example (similar to first image):
If I may make an guess, i think yours is underexposed, and maybe overdeveloped (push) symptoms. How do i know ? you blacks have little shadow details, your whites are contrasty. if you exposed at 400iso, may not be a bad thing to test your in-camera battery against another light meter. i once had 4 rolls done with a Olympus XA with a failing battery, every roll i developed got darker and darker...LOL.
But frankly, if you look at classic B&W photos, you would find that your picture is perfectly acceptable. I wouldn't worry too much about it.
raytoei
ps. just to really confuse you further, try this link
http://www.ephotozine.com/article/assessing-negatives-4682