During bowel movement, severe pain in pelvis/back on both sides in area of si joints. intense, deep burning sensation - still here 6hrs ltr. ?

Unusual: but the innervation patterns in this area are complex. If this is the first time it has happened, I would use a heating pad or sit in warm water and see if it doesn't go away, while starting a bulky fiber diet and taking a stool softener so you won't have to strain at all. If it keeps up, you may need to have an exam to be sure you didn't "split" the inner skin a little. You can take Tylenol.