Hi, my 7-month-old has been rolling over for over a month now and has started rolling onto her tummy while she sleeps. I always put her to bed on her back, but I sometimes find her on her tummy with her face turned to the side at night. At this age, should I be rolling her back onto her back or is it okay to leave her to sleep on her tummy? I just get a bit worried while I’m sleeping.

Red Nose recommendations are to “always place baby on back for sleep” not baby must always sleep on their back.
https://rednose.org.au/article/red-nose-safe-sleeping-recommendations

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 

Is your baby now in a cot & unswaddled? Babies need arms free & the space of a cot to roll well.
Ensure mattress is firm & flat (not elevated)

Does she have good head/neck control & is she mobile when awake?
Is your baby able to roll well from tummy to back (both ways)
Does she change her position when sleeping?

Have a look at the article linked & if she is able to roll well both ways, still place her on her back for sleep & let her find her position of comfort.