Welcome to HCM
You have most likely developed back muscles spasm may be due to wrong posture and possibly precipitated by cold.
A simple analgesic
and local hot fomentation will help you to reduce pain.as if muscle relaxant
drugs may be required which you will get only on prescription.
If condition is severe enough then visit to ER is advisable.
As you are over weight and a spine disc problem may be there so a x ray
spine to see condition of spine if required a MRI may give valuable information.
Please continue to take medication for thyroid