What causes "fluttering" sensation in the abdomen, both sides, lower to mid? i've some anxiety lately. feeling comes & goes, eases if i burp a bit...

See doctor: See your pcp for exam/evaluation to rule out medical causes for 'fluttering' and gassy feeling. Anxiety can exacerbate some physical symptoms, so ask for a referral to a mental health professional if you'd like that help meantime. Best wishes.

Stomach sensations: The gut is sometimes referred to as 'the second brain' because the limbic system(emotional control center) and the digestive tract are a feedback system of neuropeptides. It is a real biochemical process but can be controlled with very effective cognitive behavioral therapy, preferrably with a therapist also trained in Eye Movement Desensitization & Reprocessing(EMDR).