Medical explanation - long overdue! Sorry, but I don't think Polly is correct! Surprised that no-ne has yet offered the correct answer.
For those of the other gender to myself, "Bollocks" are testicles. If they get inflamed, you then have "orchitis" or even "epididymo-orchitis". You can get this from such bugs as chlamydia, gonoccoccus and others.
"Bollockitis" scans better in the song - you need the 4 syllables, rather than 3 or 8!