Hi girls! I'm reading here about uterine tone. Until now I didn't even…

Hi girls! I'm reading here about uterine tone. Until now I didn't even know that when the belly hardens that's uterine tone, or did I misunderstand?? Throughout my pregnancy I remember that around the second trimester and in the third my belly sometimes hardened—well, when the bump was already pretty big. But it didn't last long and didn't bother me, I thought maybe the baby is lying on its back so it's hard. Now after reading the comments, I realize that's actually tone. You read things online and it gets really scary. Is it that dangerous? Labor's coming soon, should I sound the alarm or is this basically a normal thing? Right now it really feels like I'm feeling it, plus heartburn, but the baby is active. The doctor didn't say anything about it, and I didn't ask either.