Anyone else get sick from meds they've taken for a while

I have been taking Lithium for more than 1 1/2 yrs over the last 2 years (I was off of it for a few months).  I have been taking it at a fairly low dose of 900mg.  The only times I felt sick were when I was at a higher dose, or switching between brands.  My usual level is at 1.1, but the most recent level reading was 1.7.  I have been feeling really sick from it.  I have had stomach cramps, nausea, headaches, and slight dizziness.  I have been talking to my pdoc, and I lowered my dose to try to get things to calm down.  We still don't know why my level has shot up.  I was just curious, if anyone else has been through this. 

My son is 9 he takes 900mg lithium at night for the past several months and he also has had lots of stomach problems ,cramps , nausea, pain, morning headaches etc. These symptoms last between 1-3 hours most mornings. Often times he just doubles up and whins and cries with his stomach. I took him to the Pedi and she started him on a prescription dose of Previcid at night after 2 months it has finally stopped all his problems. She said it was causeing him a build up of acid reflux she must have been right cause he 100% better now. Eating is back, he had stopped eating and was losing weight. He does take other meds for his BP as well. You might try over the counter meds for stomach or ask your Dr. about your symptoms. Good luck and I hope u feel better soon.