Ultrasound tomorrow!!Need advice please!

Suyapa • Just a cute nerdy lesbian couple trying to conceive a beautiful rainbow baby(or babies) ❤️
 I had my one month Paraguard appointment today to see if anything a moved. And well my Doc tried as hard as she could to find the strings. No luck.Its been 11+days that I haven't felt strings (myself) and I was waiting for this appointment since I havent had any pain but mild cramping. I now have an ultrasound appointed for tomorrow. 
My question is, what should I expect from this ultrasound tomorrow? Why do I have to make sure I have a full bladder? Will anything hurt??