She was just probably keeping herself very clean until now. Every bitch is different, and every season is different. My 3 year old usually bleeds very heavily but this time I only ever saw one spot of blood ;-) Don't worry about it. Easier said than done, I know, but as you don't intend breeding from her just keep a close eye on her and let your vet know that she is in season and find out when they want to have her in for her op.