Toddlers: Where is the line between shyness and something deeper?

My three year old stepdaughter will not play at the park if there's other kids around. She will cry and want to be held. She won't talk to anyone. Hubby and I take her to see her grandma at work (she works at a store) and she'll give her candy. She won't talk to her grandma at all. She will ask her questions and DD will just stare off and not look at her, and just nod her head. Like grandma asked what she had for breakfast and she didn't answer, so she started listing things, like pancakes and she'd stare off and just nod to everything she said, even though she didn't have any of those things for breakfast. My cousin came in from out of state to stay for two days with us and when she got there, DD completely stopped playing and sat beside me on the couch and literally covered her face with both hands for over ten minutes until she started to sweat. I'm just concerned that isn't normal. If she goes somewhere and someone talks to her, she'll literally avoid them, not look at them at all. She is fine around me and daddy but no strangers or even her grandma and grandpa. She's great around us, except if my husband is trying to feed her or sometimes he'll tell her to do something and she'll just stare off and ignore him. Is it just shyness or something more? I want her to be happy and healthy but it's concerning how she acts and her mom wants to start her in preschool this year at four, but she isn't potty trained yet.