J.K.
When my daughter was little she also didn't know how to express her frustration and anger. What worked for us was 2 things: First, she was breast fed and breast=feeding was used for comforting and resting. Second, we found that taking a bath and playing in the water was soothing and distracting. I think its important NOT to ignore this behavior, but to teach the child positive ways to deal with frustration by redirecting and rest. This may be her way of saying she needs to take a nap. Napping with mom or dad is comforting. Toddlers may need more rest and sleep to avert frustration.