Can a seven year old girl get her period??

She has not develped breast yet but has blood discharge

Girls are getting period earlier but this needs to be investigated. It is very early. Most girls start around 12 years or so.

It's called "precocious puberty."

Make an appointment with her doctor. The doctor should run a blood test to confirm hormone levels. It could just be early, but it could also be caused by a tumor. Get it checked. They can delay puberty and there are some good reasons to try to prevent this now if possible. These reasons include psychological development, she's probably not ready for this, as well as physical development. It can affect her height.

I hate to say this but there is also the possibility that it is not a period but that there was trauma. That needs to be ruled out as well.

Please let us know how she's doing.

While it is not common it can, and I will assume from your post it has?
A proper doctor check up is called for