I overextended my neck backwards 2 days ago by accident and heard a pop/crack. i've had mild neck pain and a come and ago headache since. i worry about vetebral artery dissection. is this possible or more likely a muscle strain?

A muscle strain is the likely result from this episode. God forbid , you experience any neurological symptoms, though it unlikely, see a doctor at your earliest. Get HPV vaccine. Wish you good health! Safe sex: https://www.hopkinsmedicine.org/health/wellness-and-prevention/safer-sex-guidelines

Don't google too much! Take Ibuprofen OTC 200 mg x 2, every 6-8 hour for a couple days, then your neck should be fine then.