I don't think that Sheila forgot to take her medication,but although she'd have felt out of control and " ill " had she taken it,she was still aware of the consequences of not having taken it.
The crux of her behaviour came after Christine had left for Canada when Sheila had time to mull over their meeting and how things could have been so different with her in June's place.Even down to playing with the twins,there were no prayers,instead,Christine joined the boys on the floor and played as NORMAL.
Imagine a thousand and one things going around in Sheila's mind as well as comparing June to Christine,which I imagine came up in the conversation on the night of the murders. This is all that was needed for such a conversation to get heated.
Sheila,with a gun,at last felt able to speak out and speak her mind about what had gone on during the meeting. The woman snapped as her mind wouldn't have allowed her to reason in the normal way.
Sheila would have held a lot of her feelings and angst inside her because as it was said on many an occasion,she couldn't talk to her mother.Sheila would confide in her friends too and was desperate to speak to one of them before the tragedy,but this friend was busy,so everything was going against Sheila,who was really desperate at this point. The poor woman must have felt as though she'd been deserted by everyone,even her father at this point,who she would be on the phone for hours to,just for someone to talk to and who would listen to her.