When you got the ceftriaxone, did you get 2 injections, or just one? If you got 2, then you got 500 mg.
Have you asked your doctor why you only got 250 mg? That's what I would do. I don't think they'd need to admit you to the hospital, but should just be able to re-treat you.
Let us know what happens. It's very important that you follow up with your doctor about this.