Brief Answer:
Please give additional information

Detailed Answer:
Hi.
Thank for your query, albeit short and specific.
To recapitulate: Male/68 - intense back pain on left side - gas pain - tried Gas ex and heating pad ...
Please provide additional information for me to assist you better:
- Since when has the back pain started? Was it associated with Gas pain too?
- Any other symptoms like bloating, nausea, vomiting, loose stools or constipation?
- Any radiation of pain to the lower limbs?
- Any Urinary symptoms?

If there is a combination of gas pain along with intense back pain, the probable cause of pain can be in the abdomen:
Pancreatitis
Intestinal obstruction
Gastric or duodenal ulcer
Left kidney or ureteric obstruction due to stone or so.

I would advise you hte following in such a situation:
Go to ER for clinical evaluation by a Doctor, preferably a General Surgeon.
Ultrasound of abdomen and CT scan of the abdomen and back to confirm the cause of pain, get a proper diagnosis so that further symptomatic and curative treatment can be carried out.

I hope this answer helps you, please feel free to ask for further relevant queries if you feel that there is a gap of communication.
Please give feedback about the clinical evaluation, and investigations report.

---