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Same thing happened to my OH time before last, never had issues before and the last time she was fine
Seriously, that's very rare. But yes, a bad needle placement is usually the cause. Unless you bent your elbow at some point after the needle went in. I fainted and got a huge bruise like that about the fifth time i gave whole blood. The doctor (it was a doctor putting in needles back in 1992) was a rank incompetent. I'd suggest another try, but it's obviously up to you.
My next donation is my 10th. I enjoy giving blood, good to know it's helping save lives. Might look into donating platelets, think I have to travel to Leeds for that though