Where you clicked stop sharing click add file to share you will be shown a browse window and hopefuly you will be able to find what you were sharing... This assuming you were in the library window. I hope that this helps.

It is possible that if you are sharing no files you will experience a falling of speed in a large download, an a number of hosts helping with the upload will be likely to stop you downloading from them.

Then again you could just be having a bad day!

let us know if this helps or you need any more information