Ritalin side effects question?? | ADHD Information

My 5 year old son was on 2.5mg of Ritalin twice a day for about a month and then was just upped to 5mg twice a day last week.  Ever since then he's been complaining a lot of stomach pain and asking for tums (what we use for "breakthrough" stomach pain in between reflux meds).  He's had reflux issues which we've been dealing with for years.  I am trying to sort out if this is related to the Ritalin dose increase or just independent reflux problems.  Can Ritalin cause stomach pain or even make reflux worse in children??