Feeling like something is crawling inside my anus (along with burning, worse at night, can't sleep), accompanied with occasional intestinal pains and loose bowel movement?

Several possible causes: pinworms (itching usually worse @ night);irritated skin from scented soap or detergent or frequent BM’s; sexually transmitted infection; psoriasis; or others. You could start with gentle cleansing with hypoallergenic soap & apply 1% hydrocortisone cream 2x/day for a few days but best to see Dr for eval if persists.