Just one on its own like that could be an ectopic, ie a pvc or a pac, there isn't always a thud afterwards. How many seconds would you say the pause lasted for? It doesn't sound worrisome to me if it was for around a second, very fleeting, but I'm not an expert, I'm only going by what I've learned from reading. Without having these things recorded on an ecg no one can say for sure what it is, but from your description it doesn't sound alarming. It's always best to check with a doctor of course.
Now that i think about it i sometimes get this while exercising as well, no symptoms, just a missed beat with no thud just back to normal