I'd say what you're feeling is the baby or another type of muscle contraction, it COULD be braxton hicks, lol. Real helpful, I know, I'm sorry.
I had this weird thing where my entire stomach would go rock hard and stay that way for like 15 minutes... then it would feel ok again... I dunno what that was all about but my friends midwife says that kind of thing happens when you get tired and it's a "form" of braxton hicks..
I say, "What the?..." cause what do I know, let alone FORMS of braxton hicks? I guess if any muscle can tense up, wouldn't it be safe to assume it can tense up only partially as well? :/ I know that the top of my belly will go really hard sometimes but that's just baby pushin' her butt of up there, happens mostly when I've been sitting for a long time I've noticed.
Either way, huh...