Hello and welcome to BYC! Glad you joined.
Her breed is predisposed to laying disorders including egg yolk peritonitis, internal laying and reproductive cancer. She isn't that old but it can afflict some young hens.
I lost a lovely White Leghorn to some kind of reproductive disorder that exhibited just what you are describing when she was less than 1.5 years old, then the second WLH when she was just over 2.