Does it move when you flex your wrist?

There's a muscle in the forearm, the palmaris longus, that doesn't exist in some people. It's an accessory muscle and it helps in wrist flexion, along with the flexor carpi radialis and ulnaris. In some people, it's simply not there (evolutionary weird shit) and it means that those other two muscles have to compensate for the lack of a third muscle to help in the movement. So potentially, if you don't have a palmaris longus, it could be that the other muscles are simply hypertrophic to compensate. I've seen videos of it before. See if you have a palmaris longus by touching your thumb to your pinky and then flexing your wrist (like an Italian), theres a tendon that should pop out in the middle of your wrist.

Otherwise, it's probably a lipoma or another kinda cyst. But it appearing when you have a pump makes me think it could just be absent palmaris longus with hypertrophic FCR, which is actually nothing to worry about and is just a cool fact