New swelling 3 months after inguinal hernia repair. 5 doctors and an ultrasound confirm no recurrence or new hernia. what is causing the swelling?

Seroma?: Could be a fluid collection under the skin (seroma), but that usually develops within about 3 weeks post-op, and subsides in 3-6 months. If the hernia was in the scrotum, fluid could be collecting there, mimicking hydrocele. Make sure to see the original surgeon, and consider a second opinion by a surgeon interested in inguinal hernia repair - check the americas hernia society website to find one.