Atrial fibrillation is defined by irregularly irregular heart rate. It also has a characteristic pattern on ECG.
As to rapid heart rate, or what you describe, these are most likely something we call palpitations. ( you may also feel that your heart is pounding hard).
This is not to mention that regular fast heart rates can be caused by many things, including anxiety, stress, caffeine(coffee, tea) , lack of sleep, smoking, poor exercise habits and endurance ability etc.
Next time you feel this sensation, ask someone to take your pulse and ask them to count number of beats per minute ( should be below 100)...and to assess if it is regular ( like a steady drum beat) vs irregular ( bum bum bum...bum...bum bum......i.e a random heart rate). This last bit maybe slightly difficult for someone untrained, so if still in doubt, just head on to your local GP for a quick check and ECG.
All the best