Help! I’ve never been to a concert before and idk what to wear?

We are going to see Sublime with Rome on October 5th. I have to buy an outfit because I’m 14 weeks post partum and instead of losing the baby weight it stayed there because I hadn’t been well enough to work out with ppd and hernias. I just started working out two weeks ago and have lost a few pounds but still not enough to fit in much other than my sweats and yoga pants and baggy t-shirts. I wear a size 17 pants and an xl t-shirt as of right now. Any suggestions of plus sized clothing for this concert? Pictures would help! Tia