Agree the best advice is simply to get an appointment with your GP and let him either get you checked out or put your mind at rest. Episodes of a racing heartbeat always need to checked professionally. It could be simply anxiety , but only a medical person can confirm this, so don't worry about your GP thinking you're over worrying. He would prefer to see you than not.
What can feel like missed heartbeats are quite common in people, these are what is known as ectopic heart beats, and can be caused by a half beat which you don't feel therefore feel is missed , followed by what feels like a delay then the second beat which is felt as real thudding beat. In the majority of people these 'skipped beats' turn out to be harmless, but only a GP can decide on that.
For several years I've had thousands of ectopic heartbeats every day. In my case it's due to underlying cardia problems, but even so in my case they're still harmless if disconcerting at times. Several times a day my heart will have a rythmn of doing a half beat every 2nd beat , which takes a bit of getting used to and can make me feel lightheaded as during the half beat the heart chambers aren't filling to their normal capacity. Made worse by the fact I'm anaemic so my brain starts to feel an effect. But I've been well checked out due to having a cardia history and informed it's all fine.
The sooner you make your appointment the sooner you'll get an answer to your question. Best wishes.