I feel like i have a bruise on my left forearm but i don't see one. it only hurts when i press on it or flex my arm a certain way.

It prrobably: is a bruise but NOT in your skin ...BELOW it in the muscle tissue (that's why the pain when you contract (flex) the muscle! Apply moist heat! Hope this helps! Dr Z

Soft tissue swelling: A slight muscle strain can cause a deep bruise as can bumping into something. If there is no significant limitation to the use, strength, or range of motion of the arm, then it will likely resolve in a few short days. For discomfort, you can ice, and use ibuprofen.