She's 8 months old and has been a great sleeper so far. That is an indication to you that she knows how to fall asleep and self-soothe in the middle of the night to go back to sleep. Now it is time for her to learn a new skill: settling herself down after standing back up and going to sleep.
What do you do when she stands up and starts fussing? Do you let her figure out how to sit back down and go back to sleep or do you rush right in? If you run to her at the first sound she makes, then she is learning that she will get your attention and you will come running anytime she does this. And quite frankly, I bet hanging out with mom who loves and adores her is way more fun than laying down and going to sleep!
Our baby does this occasionally still, but he will stand up, press the button on his aquarium, and eventually lay down and go to sleep. Early on, he would scream and cry but we were very consistent with our expectations (it is bedtime, thus it is time to sleep and not play). Now it's kind of funny to watch him from the crack of the door because we put him down, he stands right up, turns on his aquarium, he does a little lap around the crib, the nests like a puppy and falls asleep.
At night - If you're not comfortable with full on CIO (because really the crying isn't because she's hurt, in pain, or in need of anything essential), give her 5 minutes before going in. When you do go in, show her how to lower herself back to a seated position. Try not to pick her up and rock her. Gradually stretch out the intervals between checking her.
During the day - teach her how to sit back down from a standing position and give her lots of practice so that when she stands up at night she can figure out how to 'get back down'.
Good luck to you!