Hmmm thats a tough one hun It does sound odd at seven, but I thought I'd let you know one of my bests friends all through primary school got her period when she was seven too. After trips to the doc and a few tests they discivered she has a rare thing which causes the body to mature early, sorry I dont remember what it's called. But from then on she had to have medication until she reached an age that puberty could start.. by the time she was eleven the meds stopped working and her period came back. Sorry that's not much help! As I said it's not common so dont worry it prob isn't even it.
I think you should try talk to her about it. Tell her it's a perfectly normal thing to happen but it is really early and you being mum need to know for sure what is going on etc..
You must be doing your head in at the mo, just try relax a bit until you speak to her about it good luck hun!!