Have lightheadedness, fainted . Gave blood same day before this happened. What is it?

Gave blood 2 weeks ago in the early afternoon. Took a bath late that same night. Felt lightheaded after getting out of the bath. Sat down. Had a drink of cool water. Thought I felt better. Got up to go lay down on my bed. Took a step or two that I remember but must have been more as i fainted and woke up on the floor at the base of the shower. Husband tells me i was out a matter of seconds. Had a bruise on shoulder, swelling on right forehead, and ended up with some bruising across nose and upper eyelid of right eye. Now i have occasional vertigo when I go to lay down or rise from a sitting or lying down position. If I happen to stretch my body when i wake up in the morning while lying in bed with my back and head arched it feels like the room is spinning fast.