Is this poison ivy or am I having a allergic reaction?

I had this red rash on my butt cheek and it's gotten a really thick red color and spotted red outside of it. The next day it was red on my lower back aswell and over the next two days it's progressively gotten redder and insanely itchy aswell as spreading to my lower stomach. The week before this happened I was in this forest type place on the side of a road. (and by forest it was really just some grass and trees) and I sat maybe a minute on a blanket we brought. So idk if I'm having a allergic reaction and should go to the doctor or just poison ivy and take some medicine from a walgreens or something. Any answers or suggestions?