It’s definitely Brightshare (BS) and no one else. Would you have another affiliate account with Brightshare?
The only other thing I can think of is were they running a player sign-up promo. Maybe a few of your players joined their Mobile casino and while you may have been in the red on revshare, the payment may be a CPA connected to the sign-up promo.
Brightshare do run these extra CPA (hybrid) promos from time to time.
Cheers
Dave