I have been getting red marks on my shoulder blade's only on my right side though no pain or swelling i only notice then when i take my shirt off and see them what could it be?

It sounds like..: ..some sort of minimal trauma/abrasion. It doesn't sound like a drug/other rash to me. The one-sidedness is not surprising. Many of us, if not most, have some mild spinal asymmetry that causes one scapula to be more prominent than the other. But the real question what could be rubbing hard enough to cause red marks? Perhaps you are very sensitive to the type of fabric in your shirt or laundry soap

Needs to be checked: It is really difficult to say what the red marks are cause by from your description.We need more information like How long the rash has been there, does it itch,are you taking any meds, any other medical condition etc.It may be a contact dermatitis or insect bite . Consult your Doctor if it persists or spreads .Or see a dermatologist