Please don't worry about the cysts - cysts on the ovary are really, really common. I had several in one ovary including one about 10cm wide and while it gave me some pain, it's gone now and all is well. Talk to your doctor about your worries - most ovarian cysts are benign and don't harm you, but of course they may want to check you out to rule out any issues like pcos.
how has this happened, I'm only 21
Firstly, nothing you did or didn't do. We humans are just not very good at things like dividing our chromosomes properly during reproduction. Miscarriage is extremely common. I'm not saying that in a kind of 'buck up it happens to lots of people' way, please don't think that! It's utterly devastating when it happens to you. It is very probably just blind chance. The NHS will investigate if you've had three miscarriages for example - I have had several friends who have had three or four in a row and then gone on to have healthy babies. It doesn't mean there's something wrong with you, not at all.
You don't need to pull yourself together - this is upsetting, waiting is torture and it is natural and perfectly ok to be upset! Do you have real life support you can lean on? If you do, use it! It's totally normal to be upset when there is a suspected issue, or even waiting for tests (I'm an older first time mum and I was just a wreck waiting for mine to come back!)
Until you have further NHS testing you just don't know if there is a problem or what that problem is. I suggest you write down the questions you want to ask them - it's so easy when you're upset to go blank and forget. Also be aware that the sonographer may not be able to answer them so ask if you can talk to someone qualified to answer your questions.
The issue may be nothing - some babies just do have a bigger nuchal fold, or it could be a chromosome abnormality or it could be a heart defect, or something else entirely. Lots and lots of heart defects are fixable now with operations and the children go on to live completely normal lives.
I'm so sorry you have to wait to find out, it's really hard on you. Don't feel like you can't be upset, but try to stay busy and distracted if you can, and seek family/friend support.