please help!!! what is this????

please help!!! a week ago I started taking a progesterone only birth control and 4 days ago I started getting really itchy and red on the outside of my vagina. there's a weird kind of gross yellowish discharge (it's really weird, like it dries out really quickly but it's sort of chunky??? sorry tmi), is this a yeast infection or something more serious?? I've never had a yeast infection and neither me nor my boyfriend of almost 2 years have and STDs or anything. anyone have any idea what this is and how to treat it?