Tummy All in Knots

I'm 13w and I thought my morning sickness had subsided for the most part. But all day today my stomach had just been kind of tight feeling, like I need to vomit but I can't. What I'm feeling today feels nothing like the sickness I experienced at 8w. I can't quite put my finger on it, but something feels off. What's going on? This is my first pregnancy so I'm a bit unsure.