Could certain shirt designs or materials be causing irritation on my baby's chest?

Yes. But fabric softeners are the more common culprits.
Yes. Yes, certain materials or laundry detergents can cause some irritation on your baby's chest. It's always best to wash your baby's clothes and your clothes in a fragrance free laundry detergent like all clear or dreft and to stay away from harsh woolen or polyster fabrics if they seem to irritate his or her skin.
Yes. Some tags on clothes have rough edges or corners, so be sure to cut those tags off. Embroidery or stitched-on patches usually have roughness on the side that touches the baby's skin, so avoid buying such clothes.
Yes. Some fabrics or materials can cause contact dermatitis. If the rash keeps coming back in the same area, try avoiding the suspect materials for about two weeks and use only sensitive skin products on the skin.
Yes. Absolutely not only the materials also the products that you use to wash the baby clothes as well as the products that you use on the baby skin.
Yes. Certain materials can as your child may have sensitive skin.
Yes. Babies generally have sensitive skin. If a shirt fabric is stiff or rough, it can irritate the skin. In addition, chemicals in the fabric from manufacturing, or from laundry soap can sometimes cause irritation. Finally, the cute little appliques many shirts have are often sewn into the shirt with a layer of fabric underneath. The edges of that fabric may protrude and irritate skin.
Yes. Most babies have sensitive skin and certain materials and detergents can cause irritation. If your baby's skin seems to be irritated and you think it might be their clothing, make sure that you are using a hypoallergenic detergent (think dreft or tide free). If the rash is getting worse or if it doesn't improve with a change in clothing or detergent, you should contact your doctor.