If I have a herniated disk, is it unsafe to weight lift?

Depends. If your disk herniation is acute (within the last few months), most spine experts suggest limiting heavy lifting until the pain subsides. When lifting is resumed, it should be gradual, starting with low resistance and more repetitions before adding to resistance. Check with your provider for specific guidelines in your personal situation.
Depends. I would take the advice of your Dr. that diagnosed your herniated disk and whether you are having physical therapy or surgery for the issue. Follow instructions and advice to avoid permanent damage.
Maybe if having . It may only if you are having acute symptoms & it further depends on the type of symptoms & on what type of lifts your are doing & the amount of weight you are lifting. You may be able to do bench work rather than floor work or even use some weight machines. You may end up doing lighter loads with more reps. You would need to be guided by your physician or physical therapist .