Does anyone know what kind of egg this could be that I found outside my house? It's the size of a tennis ball,?

Does anyone know what kind of egg this could be that I found outside my house? It's the size of a tennis ball, and is just as round in shape. At first, I thought it was a gray-colored ball so I kicked it lightly to the neighbors property (no more than a foot... very light tap) and could feel that it was squishy/soft. Also, I could see that there was blood inside the egg as it rolled. It appeared that a small worm-like/slug-sort-of-thing was in there. I didn't want to go too close to it since I wasn't sure what it was and I was in a rush. The egg did not appear damaged so I'm not sure that it fell from anywhere. I like in Philadelphia, PA so the only thing I'm used to seeing are bird eggs. Please let me know if you know what this is, if it's a bad type of animal that I should be aware of.. or whatever lol. Thanks!