Menstrual Cups - Divacup, Mooncup, Instead, Lunette, Miacup

I just got a Diva cup because I love the idea of it being reusable and hate hate hate cotton pads and tampons. I've read a lot about it (folding, trying it out before period, etc etc) but I wanna know if there's anything specific I should know before my first time trying it out. It'll also be my first period in 3 months since I've been skipping it with birth control (because ew pads and tampons). Is there anything I should know?
Also, I am very prone to yeast infections. Should I take special measures with the cup?
I got the size 1 since I'm 19, not a virgin but never been pregnant.

Also, I sometimes see menstrual cup horror stories. Should I worry?

(also, stupid question, but can it fall off when I poop?)

EDIT: For some reason when I add tags they don't show up once I save the post, I'm not sure why, I'm so so sorry :(

I have a size one Lunette menstrual cup I had horse riding in the morning and I rode multiple horses (Jumping, flat work, trail riding) and had no leaks at all. Then this afternoon I emptied my cup re-inserted and jumped in the pool for 2 hours, when I got out and went to shower there was the beginning of a leak (diluted blood on the toilet paper) and when I pulled the cup out a fair amount of water was in there! Why did this happen! I am also going swimming tomorrow so I need to know what to do. I am 13 and this is my first cycle using a cup.

There has also been more blood than usual as I though I was coming to the end of my period, could this be because I was doing flips and jumps etc. in the pool?

Thanks so much
Sammi x