the threads in woven fabric go horizontally and vertically. warp and woof. i can never remember which is which. :lol:
the fabric will stretch a bit if you pull on it acros the horizontal weave. it won't stretch if you pull on it vertically.
when you cut into the fabric at an angle, that cut edge is said to be "on the bias". very stretchy. very easily distorted if you don't match and stitch carefully to the other piece. (for example, a too small triangle can easily be stretched so that it appears to match a larger block or piece of fabric.)
i always starch the heck out of any fabric i plan to cut and stitch on the bias. i'm very careful to lay the pieces together. i make sure i don't push or pull on them as they feed under the machine foot and needle. i'll pin if necessary to make sure they don't get wonkified.
there are many different ways to guard against the accidental distortion. i'll bet you get lots of good advice here. :-)