In 1930, Diego and Frida moved to the United States because Diego had been asked to paint some large murals. Frida did not enjoy living in America, she longed to be back in Mexico. In 1933 Frida and Diego moved back to Mexico.
This page contains resources which include information about Kahlo's time in America.
This website looks in some depth at Kahlo's Self Portrait Between the Borderline of Mexico and the United States.