Gave blood - lump in arm?

[VforVienetta]

I gave blood a week ago (Halloween), and have done so for 20 years.
This time my donation site has a small, firm, slightly sore lump under the skin - any ideas why??
It feels like the vein has swollen and hardened?
The lump has got a bit smaller since donation, it was peanut sized, now it's more like a small pea.
Weird huh? Just wondering if it's ok I guess.

Most likely a haematoma( blood vessel prob nicked when needle removed ant its bled under the skin)
Is skin discoloured?If you've given blood several times( at same site) it could also be scar tissue
If selling is going down I wouldn't worry, just press harder after needle removed next time and don't bend arm

Brilliant! Thanks!
Yes, she said she'd bruised me and apologised, it did colour a bit but not badly. Have had larger bruises before.
I thought I'd kept pressure on it for ages, but perhaps not long enough eh.
Hoping it's not scar tissue, I'm Left Arm Only and don't want to stop donating.