Can you be allergic to cranberry? If you are, does that mean that you are allergic to all fruits?

Yes and no. Cranberry is not a common allergen, but in medicine, anything is possible. Allergy to one fruit does not necessarily mean an increased risk of allergy to any other fruits. However, it is not common for people in their 20s to develop new food allergies; you should discuss your symptoms with your primary care physician or your allergist.
Yes. Yes, you can be allergic to cranberries but that doesn't mean you're allergic to all fruits. Allergies are to specific proteins. Cranberries and bananas have different proteins for example and maybe you're allergic to a protein specific to cranberries but not one shared by cranberries and bananas.