Keep your enemies close!
In my undergrad I had an issue with a student who I considered my "friend". We were in a small, 15 person class together. I would talk to him, with my walls down, about my ideas about the class or things that I wanted to contribute. Soon enough, I saw that he began using my ideas. At first I let it go because I thought that maybe he just liked my ideas a lot and was excited about them or he was frustrated that I didn't bring them up right away inc lass so he figured he would. After a point, it got annoying...then frustrating..then really upsetting. I stopped telling him my ideas and instead emailed them straight to the professor OR brought them up in class when I was sure everyone was listening so that he could not claim my ideas. It's tough...but I learned that people will try to steal your ideas in a heartbeat if you let them. Be private about your conceptualizations, make sure most of your correspondence is in email (so you can prove it was your idea), and make sure you share your ideas in front of a group including the professor so that way there is no discrepancy in who said what or whose idea was it initially.