What could be the cause for bruising on thighs of a 3 year old ?

My son, 3 years old, has for a couple days been complaining his belly hurts. Then at dinner tonight he said his back hurts. Then my husband was giving him a bath and called me to the bathroom to show me that he has bruising on both legs on the inside of his thighs and around to the front and bruising kind in a half moon under his belly button. We got him washed and out of the bath. It doesn t seem as noticeable once out of the bath, but it is definitely still there. He doesn t say the bruises hurt. We can t think of anywhere he could have gotten that kind of bruise. Should we take him straight to the ER or wait till we can get him into his pediatrician? What could cause this?