First off, if you're coughing up blood, you should defeinitely see a doctor. Actually, you should have already seen one. You should have done that the first time it happened. Coughing up blood can be symptomatic of much, much larger problems like tuberculosis, emphazema or lung cancer.
I don't see how I could possibly have any of those things, and I've always assumed, and my dom also, that it was just something weird with me laying face down or something. Actually, it happens in gym too when we have to run the mile. I don't know what's up with it, but if it was serious, I'd be dead by now, surely.