Irregular Fetal Heartbeat?

Hi, ladies. Hopefully someone can ease my worries, because right now I am a nervous wreck.

I have a fetal doppler that I use a couple times a week. I bought it when I was pregnant before, but ended up having a miscarriage and having it now is comforting to hear the baby's heartbeat. I was listening to the heartbeat this morning and it sounded irregular, like it had an extra beat. Mind you, all the other times I had heard it, it sounded normal. At my 8 weeks US I didn't get to hear the heartbeat, tech said it took too much energy? My 12 week appointment is coming up in a few days, so I will bring it up to them and they will probably hear it when they do the US. My question is... have any of you experienced this, this early on and had a healthy baby? Does it go away on it own? Is it something I need to worry about?