I understand that the word シクシク is 擬声語/giseigo/onomatopoeia equivalent to the English "boo-hoo/boo-hoo-hoo."
恐怖でシクシク泣く To whimper in fear
彼女はボーイフレンドに捨てられた時、あまりの惨めさにシクシクと泣いた。 She wailed in misery when her boyfriend dumped her.
However, I also see it as a qualifier for pain, but I can't tell what kind of pain it describes. Is it a pain so "gripping" as to make one sob, or is it a "dull" pain (which implies something more to be inconvenienced or annoyed by)?
シクシクと痛みます It is a dull pain.
胃がシクシク痛む have a gripping pain in one's stomach