I seed my yard twice a year as I'm suppose to,

Say what? A lawn is normally seeded once to get it established. Then,
it may need some spot seeding, maybe once every couple years to repair
areas damaged by insect or disease. But if you have to seed it twice
every year something is very wrong.



I also water and fertilize as
directed.

Directed by whom? Someone from the local agri service or someone from
a company that wants to sell N step fertilizer?

Lately I'm noticing the grass blades in some areas are no longer
the nice tall thin blades, but more to the short fat blades. I'm
concerned
because A: it doesn't look nice and B: I'm hoping its not some sort of
weed
growing. Can anyone offer me some advise?

How do you expect anyone to be able to answer this without knowing what
kind of grass you have or where you are located? Without at least a
picture, it's just about impossible to tell you what you have. And
it's important to get it right, because the solution depends entirely
on what exactly you have.