D., she is doing what is normal of most kids, with my oldest now 19, we put him in a big bed (they didn't have toddler beds then, that I was aware of) Anyway, we put him in the big bed at 18months... no issues... but we co-slept with my DD till she was 3 1/2 as she was then ready to move into her big girl bed.
I would put the mattress on the floor, I am assuming many of the other moms would agree with that. Keeping the crib up is just an accident waiting to happen.
You can get the Toddler bed but this is the age where she is dreaming and getting frightened, watch too what she is eating or drinking in the evening as this may be something to trigger being awakened.
There are a million different reasons, I would just talk to her maybe do a sticker chart and a reward. Short goals like maybe 3 days and get a reward and then after about 4 wks go to 7 days and then a reward....
This will help re-enforce what you are trying to teach.
We had issues with this even with my now 5 yr old in April, May and finally tried the sticker chart and she would earn $2 per week if there were 5 or more stickers... it only took 2-3 wks for her to relearn to stay in her bed...
She just too might be needing reassurance for whatever reason.
Remember D., to do what is best for you and your family.