My nearly 10 week old lives to stretch and arch her back. She is finally sleeping 3 - 4 hour stretches at night now that we've found a swaddle up works for her. I woke up to find that she had moved to the top of the side sleeper and instead of being vertical she was horizontal. What do I do as she still has a stake reflex that walked her up if her arms are out.

It is not uncommon for babies of this age, or younger, to wriggle while in their sleep space.
Some babies prefer to have their arms free to touch their face & suck on their fingers. These babies are wriggling in an attempt to get out of their swaddle.
You could try settling baby with arms free & see if that helps.

Also, she may be attempting to roll…... This can start to occur by 12 weeks/3mths (sometimes earlier)

If she doesn’t settle after placing her back in her position in bassinet, it may be safer to settle her as if she is attempting to roll.
The recommendation then would be to cease swaddling/wrapping & transfer her to a cot .
Babies need arms free & the space of a cot to roll well.

“Modify the wrap to meet the baby’s developmental changes, e.g. once the ‘startle’ reflex begins to disappear which is usually around 3 months; or when baby can roll (whichever comes first)”
https://rednose.org.au/article/safe-wrapping-or-swaddling-when-to-stop-and-how-to-select-a-safe-sleep-bag
Still, always place her on her back for sleep - even when rolling.(see link to article below)

Most babies will be rolling over by 4- 5 months. Some earlier, some later.
This is very normal as babies grow & become stronger.
Many babies then prefer to sleep on their side or tummy.
This is not something that needs to be stopped. It is important to make the “new” environment as safe as possible.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

How does she go with tummy time when awake?
It is also recommended to increase tummy time when awake to help build upper body strength to assist with safe rolling.