Feeling like a crazy girlfriend...

So I have no idea why, but every time,my boyfriend goes out with his buddies I get very upset and crabby and I cry over literally nothing! I don't understand it becaus ei have no reason to be mad at him when he goes out, he only does it like literally every 6 months or so. And I know all of his friends very well, I know who he's with and what he's doing. I just don't get it! Why does it make me so upset?! Is it because he's always with me every night?? I feel like such a horrible girlfriend because I nag at him the whole time he's out :( I try not to but I can't help it! Any other ladies get like this?? :(