Why is my girlfriend such a jerk?

So me and my gf have been dating for about 3 years now. And just recently (past two months) she's been yelling at me about everything. Every time I present an idea she belittles me and tries to totally undermine my confidence. She has even made me cry a few times. And she tries to control my life too.Like She tells when I can hang out with friends and who I can talk/SMS. Any advice?