Large Red Swollen, Itchy Rash On Left Buttock. What Could Be It?

I have a large red swollen and itchy rash on my left buttock. The rash covers most of my buttock and appeas to be spreading. It itches like crazy and hurts a little when sitting. I also have a similar but smaller rash on my right arm but on the underside just below my hand . I do not know if the two are related but this rash also itches a lot. I do not think it is poison ivy or oak as it does not look like it at all and I have had oak and ivy several times so I know what it looks like. I try not to itch the rashes but like poison oak or ivy it seems to spread if I do itch. I have been working with chemicals, it has been very hot outside, and I do wash my hands often.