What are the best stretches for sciatica? pain goes from tailbone through left buttocks down leg into left ankle. is heat or ice better?

Hamstring stretches: Sounds like pressure on a nerve root in your back probably L5 or s1. Stretching the hamstrings may help but i recommend you see a physician and have imaging of the lumbar spine.

Sciatica: There are no real stretches for sciatica as that is nerve compression. See your physician or neurologist before you do anything to make it worse.

Core Exercises: Typically core strengthening exercises are the best to help relieve pressure and forces in the spine. I would try and google some or look them up on youtube to see descriptions. If you can do them yourself, wonderful otherwise ask your doctor to get a referral to a good physical therapist for further assistance. Ice recommended for 1st 48 hrs, then ice or heat is fine.