HELP – Bulging neckline! [Burdastyle.de 116-042013-DL] Sewing Machines Parts and Sales in Los Angeles

I am currently sewing this dress

This is what I have so far … It’s not sewn yet, just pinned (ignore the straps). The neckline fit’s rather good (at least for my standards) but as soon as I move my arms in certain ways (in front of me/up) the neckline will bulge and you’re able to see everything down to my bellybutton 🙁

I have this problem quite often with similar necklines (different patterns) and the fabric is always very similar (rather stiff, non-elastic) and this pattern also recommended a rather stiff fabric (with or without elastane)… Maybe I set the neckline too low? But it always looks awkward setting it that much higher (high enough that the you won’t see as much, but the bulging still happens) and the neckline on the model doesn’t seem that much higher/lower than in my pictures…

Do you have tips or tricks for me? I don’t know what causes this and I’m at my wit’s end 🙁