I am experiencing a deep pain in my left quad while running & working out. long distances and deep bends/stretches increase the pain. there is weakness too. can't figure out what it is. any ideas?

Connective/muscular: Two possibilities; muscle tear, strained tendon. Pain near a joint more likely connective. Anti-inflammatories or otc pain relievers may be useful, reduced activity likely more effective. Also, onset (fast/slow) may also suggest muscle or connective respectively. If pain in calf when toes extended upwards, a more serious condition may be present. Good color and warmth in toes also good sign.

Many possibilities: Deep pain is concerning to me. That implies a stress fracture or something structural with the bone. I believe that you need to be evaluated first. Then, if all is negative, overuse and tendinitis would be a possibility. I have seen a frank quads rupture, but MRI wasn't necessary to confirm as the hematoma was huge. Referred pain from hip/pelvis also a significant consideration. Spinal cause also.