Congratulations on getting through the cancer treatment! I’m sure you’re just glad it’s over and keen to get back to normal life, including donating blood again.
Transfusion medicine has come a long way in the last hundred years. Today, donors go through extensive screening protocols prior to having their blood drawn. Afterwards the blood itself is processed, separated into its component parts (red cells, platelets, plasma, etc.), and tested for virtually every communicable disease before being released to blood banks. If you've ever had a condition that would make your blood unsafe to transfuse into someone else, the stringent systems in place will identify the issue and prevent the transfusion from happening.
Chemotherapy drugs, which generally reduce the ability for the body's cells to replicate, wouldn't be ideal to transmit to a patient who's already in a weakened state. However, these medications are cleared soon after such treatments are done, so prospective donors shouldn't let prior chemotherapy treatments dissuade them from donating. This goes for any other medical condition you think might be a barrier to donating blood. If in doubt, just sign up. If there's a reason you shouldn't give blood, the friendly staff at the lab will advise you that this isn't something you should do. Worst case scenario, abnormalities will be detected in your blood and it'll be discarded. So don't worry about whether you're able to donate, just do it - the system will sort out the rest.
Thanks for your interest in saving a life!