What to do for infant 5 weeks who is constipated. Breast fed with rice cereal in pumped milk for reflux, also on Zantac (ranitidine) two times daily. Has not had bowel movement in two days and is very fussy

Breast fed baby. Your babys " constipation" may not be related to the rice cereal at all. Breast fed babies can go a whole week without having a stool and are not considered constipated.
Stop the rice. Stop adding rice cereal to the pumped milk (good job on breast feeding, by the way!). Keep breast feeding and keep the zantac (ranitidine). Rice cereal is very constipating! For reflux elevate the head of the crib. In addition to constipation, rice cereal before age 6 months is related to obesity in later life. You may also try a glycerine suppository (found in the baby section). After 7 days call md.