I agree that these are most likely premature atrial or ventricular contractions. Typically all electrical impulses originate in the sinus node and travel in a well defined pathway to the remainder of the heart, leading to muscle contraction and thus a heart beat. PVCs and PACs occur when heart cells other than the typical pacemaker cells generate an electrical impulse and then a muscle contraction and heart beat. These may occur rarely or in some cases very frequently. Similarly, some people aren't aware of these while other people feel very symptomatic with them. These commonly occur in patients with normal hearts but evaluating for any structural heart disease with an echocardiogram is appropriate as well as making sure that electrolytes are within normal limits. If these cause significant symptoms then medications such as beta blockers or calcium channel blockers can be used. Rarely, more aggressive measures are needed such as antiarrhythmic medications or catheter ablation. Rest assured though that these are very common and manageable. Best of luck.
I would add that having worn a holter for a month, your doctors should have a good picture of what your heart is up to. Pacs and pvcs are apparently easy to identify on a monitor and I sure if they had seen anything that was a serious cause for concern they would have arranged an immediate cardiology appointment. The first course of treatment is usually beta blockers, but even then in the UK that is only suggested if patients are suffering with the symptoms, usually they say just to ignore them. They don't seem to count them as frequent unless you are having thousands a day. I was having 30 a minute in the ER and the cardiologist wasn't concerned and sent me home. Let us know how you get on at your appointment.
If your diagnosis is pac or pvc - it means you are having heartbeats that originate in a different part of the heart than the normal heartbeat. These can come from the atrium - pacs or premature atrial contractions, or from the ventricals - known as pvcs or premature ventricular contractions. The beat from here comes fractionally earlier than the ones from the hearts usual pacemaker cells, and as such the heart fills with a little extra blood which needs a slightly bigger heartbeat to push it through. This is the thump or flip-flop sensation that people feel as a palpitation. Many people on this forum suffer from pacs or pvcs, including me. Many people have thousands a day. In most people they are benign but they are very frightening at times - they have certainly caused me terrible anxiety. You are quite right to have them checked, but hopefully you will be reassured because they are very common and often caused by stress, too much caffeine, etc.