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.
She could just be a really shy person, or she might be self conscious. It doesn't really mean you're doing anything wrong... it just means she's uncomfortable. Try to be patient with her, and do things to help her feel more comfortable with you. Girls love touch, so hold her hand...hug her... tell her sweet things like she looks pretty. It may take her a little while to get comfortable with you so just wait and see how it goes.
im not the type to get nervous around guys, but my best friend always gets nervous around her boyfriend because she wants to be liked even more by him and is afraid to be herself. maybe that's the case? idk.. but hope this helps.
She's shy because she likes you a lot and she doesn't know how to act around you just yet. Ypu should take this as a compliment. Just be yourself around her and let yourself be open and she will follow you and do the same. Good luck!