She wanted to call her sister to cry over the phone, but she shouldn’t. Tonight was Sherri’s birthday, and she knew Sherri’s husband was going to give her a diamond pendant as a present.
Here, you’re “telling” the reader about why she can’t call her sister, but sometimes a little mystery is good for the reader, to pique their interest and keep them reading.
Also, if you’re in the character’s deep point of view, she wouldn’t “tell” herself why she can’t call her sister, she’d already know and would only mention it in a way that would be cryptic for anyone not in the know. For example:
She could call Sherri, have a good cry … No. She’d ruin everything if she called Sherri tonight.
And just leave it at that. It creates an aura of mystery that makes the reader wonder why tonight is so special, plus you’re in deep point of view, which draws the reader into her dilemma. Later, you can reveal Sherri’s special birthday gift to explain why she couldn’t call and ruin her birthday.