Tachycardia means specifically a rapid heart rate. It doesn't sound at all like this is what you are dealing with. Rather, it sounds like you notice that sometimes your heart rate is irregular or that, potentially, it sometimes seems to skip a beat.
In younger adults one common cause of these symptoms are what is called a sinus arrhythmia. Most of the time, this is a this is just a normal variation of the heart beat, and it is not something that needs to be worried about.
A second common cause is premature beats, which cause the heart to 'skip' periodically. Most of the time premature beats are also not a sign of underlying heart problems, and they can be brought out by stress or by caffeine consumption. On the other hand, sometimes premature beats can be a sign of scarring in the heart or a more serious rhythm problem, although this is uncommon in the otherwise healthy heart.
In older adults, another additional cause of irregular heart beat is something called atrial fibrillation, which is a chronic rhythm problem in the heart that needs medical treatment to avoid a risk for stroke from blood clot formation.
Regardless of your age, you should have these symptoms evaluated by either your primary care doctor
or a cardiologist
to make sure that no additional workup or treatment is needed and to rule out a serious heart problem.