transvaginal ultrasound!!!

i have the WORST periods ever. head to toe cramps, bleeding is weird, cramps come and go as they please, sensitive boobs, blah blah blah. the whole shabang. i have seen a gyno before and they told me i probably need a transvaginal ultrasound and my mom has told me the same thing. however, i am 15 and i’m still a virgin. the idea of anything going up my vagina terrifies me. so i guess i don’t know what to expect and i need some reassurance on what it is like.