Is it normal for blood to be in my pants after I shave off my butt hair?

Possible. When you shave your hair, sometimes- especially if you are not very careful or you can not see the area very well etc- , you can also irritate your skin and makes it bleed a little bit from the irritation. So, yes, it is possible to happen. If possible, have somebody look at it, to confirm that to make sure you dont miss anything.