What should you suspect first if you develop bruises without any reason?

It is now much less intense.

A few days ago, I noticed a bruise on my leg.

I don't have any memories of hitting anywhere. Maybe I just don't remember... haha

Since pressing doesn't hurt, I don't think it's a bruise caused by hitting something.

Actually, a small bruise has also appeared above my knee.

I don't know the reason for this either.

What is the most likely disease to suspect first in this situation?

And I think it's a bit excessive to go to the hospital just for a bruise.

What other symptoms should I look out for?