No information about the powder/ its content is mention in the question, its impossible to guess what you are using as to many of them are available ,but I will still try to answer taking it as general case scenario hoping it may help!!
Here is the thing the most weight gainer contains protein substitute in them as its major component to make body muscular, the most of the proteins
used are extracted / derived from animal meat, animal being related to human genome
bit closely then plants in case of DNA and chromosomes composition, the human immune system mostly attack mostly on the animal derivatives taking in to consideration as foreign protein (you might heard of transplant rejection/ or at least came across the term that the organs need to be match generically or may be from same family , its because of the immune rejection / identifying protein as foreign our immune system will react and will give rash
or other allergic reactions)
In short you may have an allergy
for that powder component .
Please stop using it and in case of having doubts please consult physician.