You are likely to have some kind of tachycardia! Most likely PSVT that classically cause sudden onset and sudden offset palpitations and during the episode, the person can have dizziness and weakness in the legs as you have described. But there are certain serious types of tachycardias also. So try to get the EKG at the time of palpitations. Or if it is not possible due to very short duration and if they are occurring quite frequently like on daily basis, you can get a holter monitoring done. But you have to understand that holter can pick this tachycardia only of it occurs at the time when holter device is attached to you. However if your palpitations are not very frequent but your life is getting affected by these palpitations, then you can get EPS done. But for all these things, first you consult a cardiologist and let him assess you clinically. Thanks and all the best.