Why does my throat hurts when I drink orange or grapefruit juice?

Citrus = acid. Orange and grapefruit juice contain lots of acids including vitamin c. So, if there are any ulcers or open sores in the mouth or throat, you can get a bad feeling. Tomatoes may do the same thing. Try avoiding these foods for a little while then, if the pain doesn't go away, see an ENT who can look inside your throat and get you some treatment.