What Causes Itchy Red Rashes On The Skin?

Hi, I am having a lot of itch all over my body and it gets worse when the weather is hot. it leads to long red rashes when i scratch. I took one Benadryl each day for two days and i was good. but when i stopped taking it the itch has come back again. this is happening for the past 10 days. Do you know what it is and the reason for the same? Thank you in advance.