Sacroiliitis or muscle spasms, pain left si joint sharp pain when moving sometimes but when doing the same movement later on no pain on and off 4 week?

It could be both: Pain in the joint and sometimes even near it can be seen in Sacroilitis.A good physical exam by a Sports medicine MD or Spine Specialist can help decide.Sometimes they can do an injection right in the office & get you to therapy. Muscle spasm seems to result in many patients with spine problems & diagnoses including sciatica which I see you have.PT and pilates can help build core to improve this.