It was recommended to me to take a probiotic for constipation and since taking i have been so nauseas and it is not improving constipation. (2 weeks) why is this happening?

???????: This could be an intolerance/allergy to something in the mix, or a totally unrelated symptom of your basic problem. Probiotics are just innocuous germ colonies not known to harm humans. When taken they can establish a bunch of "happy/innocent" germs that can push out the less helpful ones. I would not consider them as likely to help constipation unless it was due to abnormal gut germs=doubt it.

Which strains?: Specific probiotic strains can help with specific health issues. B. lactis and L. casei have been shown to be beneficial for constipation. See: https://www.healthline.com/health/probiotics-and-digestive-health#how-to-use