Reason For Pheynetonium Level To Fluctuate?

I take Phyentonium Sodium and Levetiricem (spelling off) to control seizures. I had a brain tumor in 1981 and in 2009 I had a seizure (2 Gran Mals back to back) and then in 2013 I had a seizure but it was was a Gran Mal. Doctor said these medications are to control seizures but they are sure affecting me in a strange way. I have a balance problem with walking, vision not good, tremor movements in the hand and my speech is slurred at times. In 2009 I thought I was having a stroke. I had a tremendous headache for about two week then I all of a sudden could not speak and walk. Doctors thought it was a stroke. I had 3 MRI s and every kind of test there was. Said it probably was some scarring from 1981 surgery. Not completely sure, but started me on this medication. Just had my Lab work done to check my levels for the Pheynetonium I m taking and it is too high (23.8) reducing it by one pill. I have never had the Leveticem checked, Doctor said not necessary. My question, why do the levels fluctuate and should I be checked for another condition and a should I have Lab work for the Levetericem I take.