How do I say it?

I want to tell my mom that I'm sexually active because I'm tired of hiding it from her. I'm 17 and my boyfriend is 19. She is very overprotective and every time something goes wrong ( like a yeast infection or a UTI) she screams and me and overreacts. I just want to come clean and tell her. (Plus I want to get tested for std's: a long story) it's stressing me out and I just want it out of the way. I can't tell her anything without her screaming at me and I just want to be tested and get this bs over with.