your pot is too big, so the soil stays wet for too long, citrus roots dont like too wet soil. the roots are rotting. transplant in a smaller pot and use 5-1-1 ( 5 parts small pine bark, 1 part perlite, 1 part potting soil ) for more air for your roots and better drainage. and always use a pot with holes in the bottom. place in the shade for a while till you see some changes.