Sorry but it sound as though you stretched the t-shirt before stitching. It is most important that any material especially one with a stretch to it, is NOT pulled tight in the hoop. Helen

You probably (would need more info) didn't use enough stabilizer. If it was a dense design you needed a pretty heavy cut-away. Sometimes, you can fix it by ironing the design as hot as you get the iron without scorching the fibers and then putting a backing on it. Like the ones used for baby garments to keep them from being scratchy. I use a product from Sulky called Cover the Back, but there are many iron-ons that you could use the same way. I hope this helps.