My heart beats so fast sometimes... And I'm short of breath. And sometimes I'm not even walking it…

My heart beats so fast sometimes... And I'm short of breath. And sometimes I'm not even walking it just comes up on me. I used to have a small heart problem so I was high risk with my first born who's now 8. After I gave birth to him they couldn't find the problem anymore. So now with this pregnancy I'm not high risk. And I have told my Dr about when I was pregnant with my first one and she tells me to just keep an eye on it and don't over due it. So I get worried sometimes. And I'm not even in the state, I get more freaked out hoping everything is OK with it when that happens also that I don't go into preterm labor lol. I'm just a worrisome person I guess