Some people's body's are built different from others. It's nothing to be ashamed of, it's just the way you are.
I strongly agree with aler7811, however. You should NOT be in a sexual relationship at the age of 13. Of course, it's your body and your decision, but you should know that you could seriously harm your reproductive organs by being sexually active so young. You're not fully developed. When you're older, though, it would be much safer, and the guys won't care. Any guy who does isn't worth your time, anyway.
Stay safe.
` Em
Your body is growing and changing, it's completely normal.
My advice to you is that if you are thirteen and in a relationship with a guy, he should not be seeing you there anyway.
When you are older though, and in a relationship, where guys are more mature, they won't care about things like that.