I'm also an A-cup and just under 100 lbs. I'm definitely a 34 band size, and the bra is comfortable and doesn't leave red marks on my skin after wearing my bra for a day.
I used to work for Victoria's Secret one summer during college. The bra band strap should
definitely not be uncomfortable and leave red marks.
Also, look down at your skin right under the armpit. (Sorry, that sounds gross.

) Is that bit of skin protruding out, right above the bra band? (As in, is the bra band so tight that it's squeezing your skin? Gosh, I hope that make sense.) My aunt had that problem for many years, and she finally learned (from me

) that she was wearing a bra too small for her.