Could rash be sign of a STI/STD?

I developed an underarm rash in Only one of my pits. It’s red and kind of itchy but bareable.

I’ve been in a committed relationship for a year now but just always paranoid about contacting something. Not sure if it’s just the change of weather as it’s been hot. I don’t wear deodorant. I haven’t changed my body care routine.

I’ve been doing research and it says a rash could be the first signs of HIV/STD 😟 maybe I’m paranoid but I don’t have health insurance and don’t know who else to ask