Anyone else with a tummy ache?

I'm in my 15th week right now! Yay! Luckily, I never had morning sickness during my first trimester. However, I've had several days when my stomach just aches. It's like it's stretching inside or something. It's kind of more in the upper part of my abdomen and lasts a day or two at a time. I thought it could be heartburn? Or round ligament pain? Or maybe baby has growth spurts? It's already so uncomfortable. I wonder what it's going to be like when I am further along?