constipation help

i went back to work 2 weeks ago. i work at a small food place that doesnt have any place for me to pump but the restroom. and im forced to use a manual pump since there's no outlet either. because of this, my milk supply dipped. i'm in the process of boosting it now but for the meantime, i've been giving my almost 6 months old baby 2.5 oz breastmilk mixed with 2.5 soy formula (lactose intolerant). in total, she drinks 7-8 oz of formula in a day while im at work. other than that, i breastfeed her. she has recently started on solid foods as well. since last week, she's been very constipated to the point where her face turns so red when she strains. it hurts her to poo. i've been giving her diluted prune juice to help her and it seems to work. she still strains when she poops though. its so heartbreaking to see her face in pain. i heard soy formula can cause constipation. should i keep giving it to her until her system adjusts or should i switch her to another formula? its just for the meantime while im boosting my milk supply. is there anything else i can do to help ease her constipation?