Found blood in 6 month old babies crib?!!?

I went into my babies room today to take his crib sheet off to wash it and I noticed a blood stain about the size of a small toy, at the end of his crib, I didn't notice this when I went to get him from his nap earlier on, but he did end up waking up with one of his legs stuck out of the bars (I was told not to use bumper pads by my doctor.) Obviusly this is pretty nerve wracking, no one wants to find blood in their babies crib. I looked him over and don't see any cuts or scratches on him, I have no pets either. Should I call the doctor? Has anyone else had this happen to them?