[Dark_Swan]

So most unpadded bras have seams in the cups. These are grat for support but i have noticed that they are leaving red marks on the skin of my breasts.
Is this normal, or a sign the cup is too small?

Does the bra feel uncomfortable when you wear it? It it does, then probably it is small for you. Try to one cup size up, or up one band size. So try sizes close to it. You could also try a sister size, that might help.
Otherwise it could be that your skin is sensitive to the fabric.