Aligners are worn for an average of 22 hours a day, except when you are eating and brushing your teeth. Patients don't always manage to clean their teeth and aligners well enough before putting them back in after eating. This means food particles can get trapped under the aligners and close to the gum. Plaque on the teeth is food for the bacteria in the mouth, and its presence makes them grow and multiply. When a large number of bacteria are in contact with the gum, they trigger an inflammatory reaction, with swelling and gums that bleed easily. If your gums bleed easily while you are in active orthodontic treatment, your first response should be to clean the problem area very thoroughly with all your hygiene tools – and always with floss – to remove the bacteria.
This answer is for general information. Your own case needs an examination by an orthodontist.