Sciatic pain, not muscle stretch, when stretching

Hello! Background: I had a herniated disc a few years ago, and an L5-S1 micro discectomy to remove a loose disc fragment a little over a year ago. I'm about 6'5", and have always had very poor flexibility, despite trying and trying to keep up, I grew very fast!
Started some sport again recently, and have been doing pilates for a few months, but! What i thought were inflexible hamstrings feels like something different. Yes my hamstrings (and infact everything from my shoulders to my heel) are very inflexible, but when i isolate muscles i feel the definite muscular stretch in my calf, hamstring, and glutes, but if i do say downward dog (yoga posture) or just try and touch my toes, i get a sharp, v uncomfortable, nervy pain. I doesn't have any of the satisfaction of a muscular stretch, and its like its desperately trying to pop my knees out (bend them, rather than dislocate them!). It is not a sudden shooting pain, but increaases with intensity as i stretch further, but becomes pretty unbearable if i push.
Its pretty infuriating, and it means flexibility progress has been glacial. Has anyone got any ideas??

Sounds like some sciatic neural tension (basically some increased tension in the sciatic nerve). When doing the kind of aggravating things you are describing, you will put more tension on the sciatic nerve (e.g. most tension when sitting slumped over, with neck and trunk bent forwards, then lifting the affected leg (or both) straight up with toes pointing towards ceiling). If you've had discal problems in the past around this area where the sciatic nerve exits the spinal cord, it's likely that could have caused it. You can do mobilising techniques to stretch out the nerve and ease the tension...

Depending on people's symptoms I teach them in different ways. If you have no pain whatsoever now (and depending on whether you had any leg pain previously), I would suggest either way you start off SLOW AND GENTLE. Sitting on a chair with your toes pointing down towards floor on one leg and then lifting your leg straight up (as if straightening your knee while lifting it off the floor)...stop when you feel the tension increasing in the back of the leg / area you describe and gently pulse at that point for about 10-15 reps (very slow and gentle..and would alter this depending on symptoms i.e. do it only 5-6x if this aggravates it a lot and unable to tolerate).

If that doesn't feel like a stretch on it at all then I would progress to doing it with toes pointing up towards ceiling...if that didn't feel enough of a stretch then move on to neck/trunk slumped slightly forwards.

Hope this helps.