The cancer story has been suspicious since the beginning since all we've got to go on is Cameron's word. She lies when she deems it necessary and what little information she provided in Gnothi Seauton was vague: "You died, December 4, 2005" and "cancer." That's it. That's the extent of the data on the death of Sarah Connor. It might've been helpful if Sarah had actually asked her more, but she didn't and Cameron didn't offer it.
Sarah did, however, ask Cameron about her diagnostic skills — whether she could analyze blood or scan someone. Though Cameron later analyzes John's blood drops on the sidewalk in Samson & Delilah and assesses library boy Eric's cancer in Self Made Man, she tells Sarah no. Actually, she sidesteps the blood analysis question and rephrases the scan question so it's specifically about CAT scans. Then she says no. So I guess she wasn't so much lying as withholding truth.
The only time I felt like we could believe Cameron about Sarah's cancer was in Automatic for the People. It was just a few days after the glitch and her decision-making was clearly impaired. She seemed less calculated, more honest, and it made me believe that Cameron believed Sarah Connor died of cancer in 2005. And when she told Sarah she didn't know if she was still going to get sick or if her exposure at the power plant would be the cause, I believed that too.
That brings me to the title of this post and that little vial Cameron pulls out of the safety deposit box in the bank vault in the pilot.
"What is it?"
I think Sarah Connor did die of cancer in 2005 and I think that cancer was just starting to develop in 1999. Sarah wouldn't know about it for a while yet, maybe even years. But Cameron knew. Based on surviving medical records and some research into cancer, she deduced that 1999 was the best point in time to go after Sarah's cancer and... kill it before it was born. 1999 was the best point in time to hand her that weapon, made by their best engineer, and make sure she exposed herself to a good, strong isotope blast that could target the cancer cells.
Whether or not firing that gun really cured Sarah is unknown. Cameron couldn't be sure if the plan would work and she was keeping tabs on her health all along in case it didn't. But that weight loss she noted in Adam Raised a Cain and Born to Run? That was a lie.
ETA: Updated this because it wasn't "in the last 3 months" that Sarah supposedly lost 11% of her mass, it was in the last six weeks (which makes it even more unbelievable). Also, it's in Born to Run where Cameron gives the percentage. In Adam Raised a Cain, she only says "she's lost weight".