How can i be allergic to ibuprofen and not asprin?

Can happen: Although rare, but first you need to define what kind of reaction you perceived as an allergic reaction? It can still be a side effect not a true allergy to NSAIDs, or a reaction to a certain brand, also what dose of either aspirin or ibuprofen you received, for all practical purposes you want to avoid whatever you believe you are allergic to, till seen by allergist