My 5 month old daughter recently had a CT scan of her brain. They said negative. What if wrong?

Unlikely. A ct scan is an incredibly detailed image of the brain. Of course a lot goes into the quality of the test, but for larger issues, they are easy to spot. With history, a neuro radiologist can look in the proper areas to spot small issues. Every ct scan is professionally read, not just by the ed or the family doc. If it was read as normal, it probably is. If there are ongoing symptoms, get an eval.
Not likely. But you can always ask for a copy of the study(possibly for a fee) and have another radiologist read it(for a fee)...