I am self medicating for ADD and have found that Ritalin has really helped improved my life.

I have been taking it on and off for a few weeks now. Today I have taken more than my average dose and have noticed three raised red bumps on my legs about the size of a 5 pence piece. They look alot like insect bites which I am definately allergic to but I am sure that I have not been bitten.

How serious is this? Can I take an antihistamine whilst continuing to take Ritalin?