felt so painful

I would write:

"A vampire bit me. It was so painful that I screamed loudly.

Or, 'it hurt so bad I screamed loudly.'

Cycloptichorn

It is strange, to my ear, to describe oneself than the injury as painful. I think most English-speakers would sooner say, A vampire bit me. IT (or THE BITE) felt so painful that I screamed loudly.

Merriam Dictionary defines "painful" as "feeling or giving pain," so saying I felt so painful sounds like I felt I was feeling pain or, even more confusedly, I felt I was giving pain. Since it is the bite that is giving pain, it might be better to say The bite felt so painful. That said, I'm sure there is precedent for I felt so painful. Most English-speakers would understand what you meant, anyway.

Thanks to all of you for your help.

Vampire bats are usually rabid. I'd have that looked at.