Hello! Thank you for the query. Appendectomy is a type of surgery which always brings risk of wound infection. Moreover, after every abdominal surgery, an abscess inside the abdominal cavity can appear. Your red bump can be also caused by stitches(under the skin there are at leas 2 layers of stitches) which irritates the tissues and gives inflammation. You should visit you surgeon, have soft tissues and abdominal ultrasound performed to rule all this conditions. Hope this will help. Regards.