My 3.5 week old started wiggling to the side of the bassinet so she is diagonal with her nose pressed right up against the side of the bassinet which is made of mesh. She always wiggles to the side she favours facing when sleeping. We put her in an ergo pouch swaddle bag with her arms across her chest. Is her position dangerous and what should I do? Sometimes she sleeps in her bigger cot and I have found her rotated 90 degrees in that too.

Many babies will wriggle & change position (like you describe) at this young age….. 
Some babies just do not respond well to wrapping/swaddling & like their arms free.
Often these babies like their arms free or like to touch their face or suck fingers, & will wriggle to get free.

Are you using blankets?
How does she go with tummy time?

You could try to modify her swaddling to have her arms out.
Red Nose has the following information:
Principles of Safe Wrapping - modifying wrap - use 3-6mths of age.
https://rednose.org.au/article/wrapping-babies

Sometimes this can mean that baby is soon going to be rolling. - not always.

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 tummy or side.

Once she is rolling it is important to make the environment as safe as possible for this to occur.
Continue to place baby on the back to sleep.
Ensure cot mattress is firm, well-fitting & flat.
Ensure there is nothing loose or soft in the cot.
Preferable to use a sleeping bag (with arms out) when rolling that has appropriate size neck & arm holes.
https://rednose.org.au/article/what-is-a-safe-sleeping-bag

Red Nose has the following information when baby is rolling.
Give baby extra tummy time to play when awake and supervised as this helps baby to develop stronger neck and upper body muscles which in turn enables them to roll back over.
Make sure that baby is on a firm and well-fitting mattress that is flat (not tilted or elevated).
Make sure that baby’s face and head remains uncovered (do not use lambswool, doonas, pillows, cot bumpers or soft toys)
As babies grow and develop they become very active and learn to roll around the cot.
At this time, continue to put them on the back at the start of sleep time, but let them find their own position of comfort.
By this stage it is not necessary to wake during the night to turn baby over to the back position
Do not use any devices designed to keep baby in a particular sleep position.
https://rednose.com.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over