My 15.5 month old has trouble pooping while sitting down. she always wants to stand to poop and tenses up. is this normal or should i be concerned?

Make stool soft: Potty training at this age can be challenging. Make sure everything is done to make sure stools are soft and the act of passing stool is not painful. This can be accomplished with high fiber foods (fruits and vegetables), increased hydration with water, and prune juice if needed. Miralax (polyethylene glycol) is also an option. Continue to encourage child and give lots of praise for attempts and actual BMs. Keep at it.