Hi
Thanks for writing to the forum!
Your post indicates you have rash and swelling in one leg only. Is that right? Was the medication you took for local application? Why were you put on a new drug?
If it was something you took by mouth, it will cause a generalized reaction. Antihistamines help to control itching and such reactions. However you must be sure that your problem is due to the medicine you took.
If you are bruising easily it could be due to weak capillaries, low or abnormal platelets, low Vitamin K etc. Swelling in one leg could be due to infection (like if you scratched and infection started through broken epithelium) or filariasis.
I would suggest you consult the doctor who started you on the new medication. I would need some more information to be able to answer your queries better.
Do let me know if there is any thing else. Take care!