Why does my back itch so bad every evening/night?

My back (and sometimes my legs) have an intense itch just about every evening/night for a few years. I don't have a rash or marks on my skin (until I start scratching, then I get welts). I bought a new mattress (thought it was bed bugs) - didn't help. Changed laundry soap & body wash/soap (thought I was allergic) - didn't help. I don't usually itch in the daytime. I am very allergic to bleach (I break out in a huge rash/welts & I get so itchy I want to rip my skin off) - I found this a few years ago at work, so needless to say I do not have any bleach in the house. Sometimes the itch is worse & I have to take an antihistamine to help me sleep, it also helps take away a little of the itch. Please don't say bedbugs - it itches before I even lay down in bed (it's actually itching right now as I type this). If anyone has similar issues, or knows what this may be, please let me know. I have no medical issues. I'm healthy & fit other than this stupid itch.