What Can Cause Cramps At Lower Back And Abdomen During Embryo Transfer?

My wife is 33, 5 feel 5 inches and 62 kg. She had an Embryo transfer on 19th Dec, in which 2 embryos developed by natural selection and well developed were transferred. She has started to develop mild cramps on the left back side and lowe abdomen with a feeling of periods getting stated.Her breasts seem tender which I understand is due to Progesterone 200 she has been taking. But am worried about the cramps and period like sensation. could anyone throw some light on the same. is it normal?