I have recurring elbow tendinitis. Some Iburofin for the pain and inflammation and one of these when you play or have pain should take care of your problem. If they don't see your doctor. When moving your forearm muscles, the pain often manifests itself on top, and that can be misleading. You want to apply pressure wth your fingers in the area on your elbow just below and above where the two bones meet - you should be able to find where the inflammation is by the pain from pressure (as opposed to movement), then make sure the pressure point of the strap is across the point where you found the inflammation. It seemed ridiculous to me that the strap works when I first started using it, but it does.
BTW, this tendonitis can often be caused by repetitive motion like typing on a keyboard, playing PS4 (guilty!) or other activities not associated with playing guitar. So think about what you do and wear the strap anytime it may apply and you are feeling discomfort. Unless you have severe pain, I disagree that you need to stop playing. You just need to support the injured area.
Again, see your doctor if the pain persists.