Pain and some weakness in outer side of upper left forearm after striking golf or tennis ball (forehand stroke only--I am left-handed). Cause?

Tendinitis likely. From your short description, it sounds like tendinitis in the L shoulder, either from overuse / over-training or from some other injury. Playing sports just aggravates it. This is by far the most common cause of shoulder pain in athletes. For now, rest the shoulder, ice it as often as you can, & sleep in a comfortable position (on right). See a doctor or use HealthTap Prime if you're not better in.