She sounds like an introvert, where your energy is drained from being around the one she is dating (due to being nervous)... Here's a link that explains what an introvert is ( link
Regardless, being shy is normally caused by many things:
- She is naturally intimidated or "worried" about losing you by acting like a dork / etc
- She has a bad past with previous relationships that have left her scarred
- She may have something to hide from you, and it's making her nervous
- She may not be comfortable with you (if it's a new relationship, this is VERY common)
Just open up with her about yourself and accept who she is. Give her the benefit of the doubt (your patience) and allow her the time to open up, it's more than likely that once she is comfortable, you will see a whole new side to her.