any of you ever experience the relationship blues?

 So I've noticed that when I'm single, I typically lose a couple pounds (not unhealthily, it just sort of happens) and overall I feel good. Don't get me wrong - I'm in a healthy two-year relationship now, but I noticed myself fall into my usual pattern and I'm wondering how to break it. Before i started dating my boyfriend, I took some time to really discover myself. I'm a very free spirit and I love myself and the person I've become. I have noticed though that as soon as a guy asks me to be his girlfriend, a small piece of that free spirit disappears. I am still me, but I think too much about things and there's totally another person that I now have to think about. Another discovery is that I start to gain weight. Not a huge amount, but I've definitely gained a solid ten pounds since we have started dating (in my current relationship). I don't know if it's me being comfortable or what, but it's frustrating because I feel like no matter how many runs I go for, it either doesn't go or keeps coming back. It seriously feels like the second that a label is put on the relationship, weight is gained and a piece of me goes missing. It almost seems like some chemical imbalance in my brain that causes this. I don't want to not be in a relationship though, you know? I also have noticed that recently, I've slumped into some blues. I'm not really sure why, but I just don't feel like the fun free spirit anymore. I feel like I really need that piece of myself back, because that's one of my favorite parts of myself. Have any of you ever experienced anything like this?