Is there any reason for choosing the intra venous injection route? Regarding my experience, for cellular xenograft, I would recommend sub cutaneous injection on the mouse dorsal flank.
If I understood well you inject lymphoma cells (probably B-cells?). Few publication (more or less recent) described a possible link with peri ovarian inflammation mediated by immune cells. that may explain a possible burst of the ovaries linked to the inflammation. Did you proceed to any IHC analyses of the ovaries?