Welcome from Ontario, Canada. You need to keep your hand on the fabric to guide it as it goes through the machine for a good 1/4 inch seam. Try making a simple 4 patch and measure the finished size to see if you are piecing correctly. If a seam opened up, it probably didn’t have a consistent 1/4 inch seam or it could be the fabric was poor quality.
Bonnie Hunter in her Tips and Techniques tab at the top of her Quiltville site shows how to make a seam guide for most machines and you can see if having something to run the fabric against, helps you do straight seams.