What Causes Heart Palpitations After Donating Blood?

i am a healthy 35 yo female (no chronic health condition), although i m overwieght (185 lbs 5 4 ) but i have being experiencing pressure in my head with occasionally slight/low grade fever no headaches, just a general sense of not feeling well, tired and all x2weeks. I am active and get regular check up every year. today i gave blood and my heart is racing. the folks at the blood donation center said the increased hr is normal after blood donation but i am worried please help