Based on all the info and pic you provided, it may be along the flexor pollicis longus or flexor carpi radialis. Could be a strain, trigger points along the fascia, or a combination of the two.
I don't know exactly how you bb bench as opposed to db bench. You say you db bench with elbows flared out to get a better stretch (a mistake IMO but that's another thread entirely) but other than that, it's impossible to determine exactly how you engage your flexors.
By the way, I remember your thread re: lower back pain from about a month ago. Did you actually try the things I suggested?
The reason I ask is that I have been meeting more than a few people in person as well online that:
a) Genuinely have some sort of pain but consciously or subconsciously refuse to take corrective action. This gives them an 'out' from meeting their personal goals.
b) For one reason or another ignore the advice some of the more intelligent people on this forum gives (for no financial reward).
AND a little bit of
c) An entitled attitude. The professionals out there who offer sound advice on forums like this really don't have to answer shit. They usually do because it's in their nature to be helpful if they genuinely believe they can contribute in a positive way. But human nature being the funny thing that it is, people often don't appreciate something they get for free.
Maybe you don't belong in any of the three categories. Yet I've learned to balance my skepticism with my instinct.
Good luck with your injuries. I'll leave it at that.