How do I switch my sleeping pills with her aspirin, when do I do it?

After she asks you to get them, go to her locker in the back near the shower (it's the only locker that's open), open it and pick up the set of pills (you'll have to move her deodorant), while you're still in CTRL mode use the mouse wheel to pull the pills close to thee camera until you get a thought bubble saying they look just like your sleeping pills.

You can drop or put back the pills, press ESC and go to city view, go back to your apartment and find your sleeping pills (they're on a night stand/desk by the bed under the painting of a burger with the buns shaped like a woman's derriere). Then you go back to the gym and put your sleeping pills down on the table beside Samantha.

Wait a while, start cleaning, then after a bit you should see her head rolled back indicating she's passed out.