Pain when crossing left leg -- Piriformis syndrome or something else?

[Apptar]

I started gym training 3 months back. May be the trainer pushed me hard with leg press or squats, I noticed limited mobility of my left leg after few days. I notice limited range of motion of my left leg when I do any of the following:

* I need to rotate the hip externally,

* I try to put my left leg on top of right leg,

* I try to do breast stroke in swimming to push my left leg out,

It's too sore to perform full range of motion with left leg. I get pain in upper thigh muscles  and pressure extending towards hip that limits range of motion.

In addition, I get pain in upper quadricep muscle (probably rector femoris) of left leg when I lie down and bring my left leg and knee towards my right shoulder, OR, if I do child sitting posture in Yoga  (https://www.youtube.com/watch?v=eqVMAPM00DM).

I have seen 4 doctors in past 3 months but no avail. The latest I am seeing doctor feels it is Piriformis syndrome which causes pressure on Sciatica nerve when Piriformis muscle is compressed. I am not sure if it is the correct diagnosis.

I did an X-ray of hip & back and they are all normal. MRI of lumbar spine is also perfectly fine with no evidence of herniated disk.