From what I understand of a hematoma, it's a result of an injury to the wall of the blood vessel- they are very common and usually resolve on their own. I'm not sure how you being nervous would cause this, unless you had yanked your arm away or something of that nature- which could have caused an injury.
I actually think this is quite normal during this process. Speak to your doctor if it does not heal or if you are concerned, but this is pretty common. good luck
They probably didnt put the needle in right. I developed a hematoma while donating due to them sticking the needle in too far, puncturing my vein in another spot other than where the needle was insurted into the vein.