I'm a venereal disease like a menstrual bleed
venereal disease refers to the fact that his lyrics are "sick". See my other explanation for the second half.
Tenshi
The line is "like a menstrual I bleed through the pencil and leak on the sheet in the tablet of my mind" and is a metaphor for writing lyrics in his head,compared to the female menstrual cycle.It also sounds like "minstrel" which was a medieval songwriter
Tenshi