My 19 Month Old Son Has A Purple Bruse On

My 19 month old son has a purple bruse on his low left side on his stomach witch keeps getting larger over the past 4 days and u can feel and see a lump . tonight I noticed a small bruse developing on his left sides on his back by his ribs just like the one on his stomach ... I know he hasn't fell or bumped into anything to cause any of this to happen should I be concerned or am I over reacting