I'm allergic to penicillin. am i also allergic to other antibiotics?

Possibly: You may be allergic to other synthetic penicillins as well as cephalosporins. Honestly, there are so many alternatives to these antibiotics that you should use alternative antibiotics unless it is absolutely necessary, then a consultation with an allergy/immunology specialist is warranted.

Some of them: Their are different "classes", or families of antibiotics. You would be likely to have a reaction to amoxicillin, for instance. Their is some cross reactivity with cephalosporins, like keflex, in approximately 15% of people. You could be allergic to other meds as well, but would not be much more likely than the general population to have an allergy to quinolones, like cipro, (ciprofloxacin) or macrolides.