Had cardiac ablation 3 weeks ago, today started having arrhythmia again should i be concerned?

Not necessarily: It largely depends on the arrhythmia that was ablated. If you had svt ablation, you may have non sustained runs of arrhythmia that will subside overtime. If you had ablation for atrial fibrillation, you may have short runs of atrial fibrillation up to 3 months, as the scars form in the heart. Best to evaluate this is to have an event monitor, to see if you have recurrence or not.

It depends: Generally speaking, atrial fibrillation ablations can see patients developed a wide variety of abnormal heart rhythms for up to three months after the procedure. If however the procedure was for atrial flutter, avnrt or avrt (wolf parkinson white) this may reflect the need for additional procedural work.