With your blood test and MRI report and considering your clinical history. It could be either Ankylosing Spondylitis ( its an auto immune disease affecting spine) which I am suspecting because of inflammed sacro-illiac joint and positive HLA B27. Or Slip disc According to MRI report of disc bulge at L5 S1 it can radiate pain in leg.
It totally depends on proper clinical assessment but i am suspecting it to be Ankylosing Spondylitis.
Next Steps
I would suggest you to consult a rheumatologist. Who will prescribe some medication which will help you in calming down immune system.
Health Tips
You can use Ice pack or consult a Physiotherapist nearby for Ultrasound or some Mannual therapy.