That right, it's THEORY TIME!
The lump Sarah finds in her breast is actually a microchip implanted in her by Kaliba and they've been tracking her every move since she awoke in the hospital in The Good Wound.
This is how Winston found her at Western Iron & Metal.
This is how Kaliba goons will find Charley's house.
This is why Sarah uses the defibrillator (to disable the chip).
This is how they find Sarah at the doctor's office.
This is how they find Derek and Cameron.
Piecing in the back story:
We know (from the call John Henry picked up) that Winston was discovered by a coworker at the warehouse after being shot by Sarah. That worker called a higher-up who ordered them to shut the place down. We later learned from Winston (in the van with Sarah) that his bosses took him to a private medical facility. So we can conclude that people from Kaliba showed up at the warehouse to save Winston shortly after the shooting.
Winston received emergency care by Kaliba people at the warehouse, regained consciousness and told them what happened. They assumed Sarah was working for someone, so they implant her with a microchip. To make sure she survived to be able to lead them to her bosses, they drove her (and her Jeep) to the hospital, stuck her in the driver's seat and made sure someone noticed her. That explains how the driver's seat of Sarah's Jeep was covered in blood and also why her medical chart said: "Patient arrived unconscious with gunshot wound."
Why didn't Winston or Kaliba people show up at the Connor house once Sarah got back home after Desert Cantos? They weren't interested in Sarah until they could establish who else she was working for or with. So they were simply going to track her every move until they could discover that info. But when she headed over to Western Iron & Metal, Winston sprang into action and nabbed her for interrogation before she could cause any more trouble for Kaliba.
Before Sarah killed Winston, he told his bosses that her son is who they wanted — that he was the one who blew up their factory. So once Sarah got away, Kaliba was going to be back on her trail again, wanting her and her son and probably everyone in their "family" operation.
What we think is going to happen:
Sarah finds a small lump in her breast and immediately assumes the worst — that this is the cancer she's going to die from. With Derek having lied about Jesse and Cameron now posing all sorts of problems, Sarah makes the decision to ditch them both and go to the one person she knows she can trust completely with John's life and the only one she can trust to finish raising John in her absence: Charley.
With John safely tucked away with Charley, Sarah heads off to see a doctor about the lump in her breast. When she's told that it appears to be some sort of a foreign object, Sarah puts 2+2 together and realizes it's a tracking device. In a desperate move to disable the chip, she grabs the defibrillator and zaps her breast, knocking herself unconscious. But it's too late. Kaliba is already moving on the whole Connor clan.
Charley's house is under siege, Derek is getting tasered, Sarah is about to get nabbed at the doctor's office and Cameron is fighting a Kaliba machine (the water delivery terminator). This is the "fatal Skynet attack" we've heard was going to happen and the most likely casualty is Charley, who will probably die defending John :(