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is done through a vein in the cubital fossa (in front of the elbow). I presume you have this pea sized lump in the same location. If this is where the lump is, it is probably an enlarged lymphnode which has developed due to inflammation following the procedure.
An enlarged lymphnode without pain or other secondary symptoms is harmless and not concerning. Most often it would go away, rarely it can persist without causing any problems in the future.
Hope this answers your query.
Wish you good health.