upside down catfish died

[mc4x4]

i have a upsidedown catfish that died last night the belly was swollen and i thought it was gonna spawn so i put it in a breeder. after awile it was gasping and i noticed a lump on the side too and after it died i did notice the eyes were kind of buldging anybody habe any ideas what it was? i have him in a 35 gal tank with other catfish and various tetras. all water tests were good. thank you for any help