My daughter is just under 5' and ~100lbs. She's flown K13s solo with wooden backrest plus incompressible foam, and confor/dynafoam under her backside.
As others have noted, having the stick tickling your stomach becomes an issue, particularly when wearing heavy winter clothing.
Different K13s have significantly different minimum weights, so it is necessary to check the individual aircraft. Make sure the ballast is securely fitted. A BGA safety flash went around a few years ago after a couple of incidents where it wasn't correctly secured.
Any gliding club will be more than willing to discuss your issues - if nothing else, turn up and ask.
Have fun, safely.