What Causes Black And Blue Bruises On Stomach And Arms?

Hello, I have just recently noticed this huge black and blue bruise that apeared on my stomach. I have no idea how it got there and by the size if it, i definaty would have remembered if i bumped into something. It is kind of sore to touch and ive had it for going on three days now. I have also noticed small brownish bruises on my arms and today while I had an extremely horrible itch, i noticed after I scratched, another blue bruise had formed. What do i do? What could ber possible reasons for this situation?