Hi leatherface,
Anytime blood is comming from there needs to be seen by a doctor. Yes it could be serious but I will bet it's probably just a fissure or a hemeroid. Get it checked out to be sure!
The doctor will ask you some questions, and may not even look at your butt. If he does, it's painless and really not as bad as one would think. (trust me, Im a self-consious female and had to have one, took every bit of bravery I had in my system to do that! But afterwards realized it really wasn't bad at all)
Does it hurt? How much blood is there? What color is the blood? (bright red, dark red) All this is important.
Honestly, I wouldn't be worried about it. Just get into that doctor and tell him/her about it. Untill then, increase your fiber intake (veggies, fruits, grains), drink TONS AND TONS of water, excersise.
Good luck I know it's scary but you will be OK

What you're going through is more common than you realize. Let us know what happens!
Take care,
Lori
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I wish to be the kind of person my dog thinks I am.