Just passing this on: if you are allergic to contrast, it is probably to the kind you drink (which in many -- not all, but many -- cases is optional) or to all of it.
If you are allergic to certain corn / soy / common fruit flavors used to flavor the contrast, ask it be made without the kool aid.
That iodine / shellfish allergies are involved is *reportedly* a tall tale.
And just as a note: if you are allergic to contrast, maybe like me you were so sick when you took it the first time it was difficult to say why you were barfing and letting loose all over the scanner and then had horrible abdominal pain / gas for two days, YOU PROBABLY ARE, but try to call and find out exactly the brand you had, and make sure it is put into your records.
Oh, and on another note: you know how they bring you five cups, whether you're 6'4" or 5'4"? Have you read how "up to 4 servings of contrast" are good? If you feel yourself rejecting the contrast after 3-ish of them, stop drinking it, and let them know you've done your best, end of story, and no you're not going to hold it and pretend to try. You've got PEOPLE to read. If you feel that your doctor(s) need the easiest-to-read CT scan to do their jobs, even if it makes you sick, GET NEW DOCTORS.
You have a little time -- not a whole lot, but a day or two -- yes.