Vania,

If you slept with your forehead against the headboard or wall, that pressure alone could cause a bump. It's like falling asleep while riding in the car with your head against the door frame and waking up with a bump and sore spot. There was no impact, just a sustained contact.

If you want to have something to be concerned about, I suggest you look at your sleeping arrangement and consider if you can change anything so that you cannot sleep with your head up against a hard surface. It could be changing your position to a safer position when you go to sleep.

I have learned what positions are best for sleep. If I follow my own rules, I sleep better and often wake up in the same position I went to sleep in.