My daughter wants everything she sees. We can't go anywhere any more. Advice please!

It's awful. My husband stays home with her while I shop because she demands everything. She is 8, so definitely knows better. She's getting in trouble at school too because she'll just take whatever she feels like. I'm constantly finding things that aren't hers in her book bag. I'm at my wits end. Discipline doesn't work and we've tried everything including confiscating her Halloween candy until she can show us she can stop touching things that aren't hers. She is defiant and lacks empathy.

She demanded a classmate's Halloween earrings on the 31st. She doesn't even have pierced ears. I've even used that as an incentive but she doesn't want to have her ears pierced.

I've done some reading online and I don't know if this is serious enough to talk to her doctor about just yet. She has lots of signs of oppositional defiance but this has only been going on for about two or three months. She does not have ADHD or any other medical/mental issues. It's purely this impulsivity and lack of empathy when she's taking items. Otherwise, she is pretty normal. There have been no big changes recently, other than going back to school.

Edited to add that I'm not questioning taking her to the doctor. I'm questioning if I should do it now, or else wait and try some other things first. This is the first time she's ever done anything like this. Of course all kids go through that phase where they want everything in sight but she was never really like that until recently. Her school seems to think it's a phase she'll grow out of and they're more concerned with just having the behavior stop. I asked if I should take her to the doctor and her teacher looked at me like I had three heads.