How do I know if I am having reactions to Lithium toxicity?

I've not had my blood checked in three months. I've been having tremors for a couple weeks. Last night I started feeling nausious. Today I felt like throwing up. Nothing came, but I started drinking water and felt a bit better. Yet I've felt quite weak all day. Not hungry. I took myself down to 900mg from 1200mg, also. I also take Effexor and Xanax.