Training Biceps

I’ve started to notice more and more now that when I train my biceps insteak of having a peak mid bi there is a little valley. Does anyone else see this on themselves? Is it just my genetics or what because I never used to notice it. Also, when I was doing bis a week ago. I finished my workout and all day my left bicep felt almost strained but not badly. The thing is though, my thumb on that arm has been twitching every minute of every day since then. Lol, whats going on?