Had bunionectomy, doing fine then bent down and heard a pop and it hurts. Whats up?

Go back to your doc. If you're in the first 3 months after your surgery, it's very important that you go back to your surgeon and let them know about the pain and pop you heard. You may have harmed the surgical site, and may need follow up x-rays to check the area.
Several things. You need to see your surgeon to make sure you have not injured the surgical site. If a screw was used it dould be disrupted or bone could have shifted depending on how long ago surgery was.
See your surgeon. There could be many reasons for this especially depending on when your surgery was performed, the type of procedure used and your post operative course. The best person to answer this is the surgeon who operated on you.