Questions

I've been attempting to do juice fasts for two weeks to clear my system and help get rid of my extremely bad constipation before I start my diet that includes whole fruits/veggies and chicken but after 3 days of going into the diet I throw up a ton of mucusy type liquid. I usually feel fine after I'm done but I'm wondering if this a sign that I need to do something before I attempt this and that theres another problem or what.

My situation that I'm currently dealing with is bad constipation/my stools are either sliver shaped or round and small, and when I eat problematic foods I produce a lot of mucus in my throat/nose either instantly or a minute or so after.