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It is good to donate blood, if you have good Hemoglobin
level there should not be any thing to worry,and as far as heart beat in head and ears may be beause of some hyperdynamic circulation you are feeling that and it should go in a day or two.
As donating blood stimulates new RBCs to form hence new RBC comes in the circulation.
Keep donating blood once in 3 to 4 months.it is life saving to some one.
Take care of yourself.