I was recently in the hospital and treated for pneumonia with rocephin in spite of a supposed allergy to pcn.does this mean im not allergic to pcn?

"supposed allergy": is the key word, but still we can't clear you to be NOT allergic to PCN, although highly unlikely that you are. Cross reactivity of penicillin and ceftriaxone (rocephin) , is much less than other cephalosporins. If you need PCN and it is the only treatment for your condition, then testing / desensitization can be performed, otherwise there are alternatives, so you don't need to worry

Maybe: it means you are not allergic to ceftriaxone (rocephin), which is in a similar drug class with penicillins (beta-lactams).

No : Rocephin is not a form of Penicillin . It is a different class of antibiotic and can be tolerated in penicillin allergic patients .