Selena Gomez, Kidney Transplant Harder on Donor Francia Raisa

Selena Gomez has been through hell and back with her failing kidneys, but we're told the transplant operation and recovery was actually more difficult for her best friend who donated her kidney -- Francia Raisa.

Sources close to Francia tell TMZ, removing her kidney was a show-stopper ... she could not do any physical activity for 2 months.

We're told it was especially difficult for Francia, an actress who starred in "The Secret Life of the American Teenager," because she's a "health nut" who has been deep into various forms of exercise and physical activity.

The operation was months ago, and we're told just now Francia's able to do less strenuous forms of physical activity such as yoga. These pics of her and her yoga crew were taken a few weeks ago.

We're told because of the transplant, Francia's diet is restricted and needs monitoring.