Hello
Pain in the left lower quadrant , radiating to the back (mild to moderate degree ) may be due to these possibilities , these include.
1 The first possibility is of left
renal colic , diagnosis can be confirmed by
ultrasound of kidney. Also get in urine complete examination.
2 The 2nd possibility is of amoebic colitis . Diagnosis can be confirmed by physical examination as well clinical history with stool examination.
3 Carcinoma ,(age is 51 years so more prone ) diagnosis can be confirmed by
colonoscopy examination ( although there is no change in the bowel habit but still we can't rule out cancer of large intestine due to referral nature of pain).
However , there are so many other causes as
ovarian pain with
fallopian tube ( in female patient).
As you mentioned that in the night you are taking maalox , while prevacid is also in the morning but these both provide relief as symptomatically .
Since you are also feeling fullness in stomach as well loss of appetite so I recommend you please consult a gastro-enterologist and get his opinion.