Welcome from NJ.
Remember when working with triangles,, there are 2 bias edges,, so sewing & pressing carefully is critical..
Don' t pull the triangle when sewingm just let the machine feed it, less chance of stretching.
When pressing, use. Spray starch, & put the iron down on the fabric,, lift & move to the next area, don't move the iron along the seam.
Hope these 2 suggestions help, don't get frustrated, even experienced quilters run inti bias stretching if not careful.