Talk to Gynecologist on Back Pain In Pregnancy

I'm 20weeks pregnant and have been having severe pain on my lower back especially on the left side. What should I do to relieve it?any medicine, ointment or should I get physiotherapy or massages I'm clueless. Kindly help it's getting unbearable. Please also mention if there's any specific exercise or sleeping/sitting postures that I should be avoiding to help ease the pain. Thank you.