Ritalin SR or Methylin ER?

I was put on Methylin ER (Generic form of Ritalin SR) and I've been having very sporadic side effects. The first day I took it, I was fine but couldn't eat one bite. The next day, I made myself eat something and my heart started t really hard. Over the next few days, I was experimenting with holding off eating so the medicine won't make me jittery. Has anyone had this problem with Ritalin SR or Methylin ER? If so, what has the doctor switched you to? Another thing I don't like about Ritalin is that it makes me anxious and my gym schedule is screwed up from it now. I can't go workout feeling like this because I'm scared I'll go into cardiac arrest.