Hello, I Have Been Breaking Out For Over 3 Weeks

Hello, I have been breaking out for over 3 weeks now with painful itchy red bumps that started on my chest and arms. Now they are all over my body. I went to the ER and they only stated something might have bitten me and prescribe me Benadryl 50mg. That only holds off the itchy for 3 hours and the itchy painful feeling returns. Please can you help me.