It's so annoying when this happens but it's all a part of being an online writer. You soon discover that not all contributors are honest people!!!
This issue is ongoing and has been thoroughly aired in forum after forum - just check out the forums to learn much more.
The Learning Center has a section on stolen content which you should read. Plus, I got so angry when some of my work was stolen I wrote a hub about - which helped get rid of some negative energy!!
I wish you well.