I am 20 weeks pregnant & for the past week, sometimes when I blink it feels as if something is cutting my eye. What should I do?

Get it checked. You may have somehow scratched your eye, you may have dry eyes, you may have a corneal abrasion or ulcer, etc. Bottom line is that most minor things would generally have worked themselves out by now, so i'd suggest you see your doctor or an eye doctor to get checked out.
Lubricate. Lubricate the eye with an artificial tear (no danger to your baby). Get it checked out if this doesn't help after a day or two! if eye is red, painful or there is a discharge, go to an eyemd for help!