I agree with kmonte - grieving is just working through your feelings and may or may not include crying. I think the lack of focus in school and the sense that you may lose control at any moment probably signal that you have emotions that need to be worked through. This can be done in a lot of different ways, from talking to someone about how you feel to what kmonte said about finding ways to feel connected to your mother, etc. Or taking care of everyone may feel like too big a burden for you to carry, and perhaps the symptoms are related to that. Regardless, you should find a professional to speak with who can help you feel better during this time - I think that would help a lot.