Maybe she hadn't packed the medication. She was a scatty character anyway, probably with no sense of time, which is part of the reason why she failed to hold down so many jobs. Barbara Wilson also noticed Sheila's gait, walking through a gaggle of geese as if they weren't there.
Part of the symptoms of schizophrenia is not being able to distinguish fantasy from reality, which is why some end up believing voices in their head instructing them to end it all. Sheila was disoriented, she was confused, which is why she spoke no words on the trip down to the White House and why this pattern of behaviour was duplicated on the telephone with Auntie Pam, not that she felt any particular malice at that point to any of the individuals concerned.
She may have forgotten to pack such important medication Steve but even if she did she had only been away from home a couple of days, I am sure there would still be traces of such medication in her system.
Sheila was highly aware of her appearance, no one can argue with that. I am sure the last way she would want to be seen was 'walking like zombie', gurning and muscle twitches to her face etc. Those side effects are horrible and so demeaning and embarrassing for anyone, never mind a former model, celebrated for her beauty to have to suffer.
I am absolutely positive Sheila would have done anything possible to hide such ugly physical symptoms ... but no meds?? strange imo.