Hi Sam,
Finally at a computer so can type this out properly:
OHSS stands for Ovarian Hyper Stimulation Syndrome is a pretty rare but serious side effect from fertility treatment. It occurs when your ovaries are hyper-stimulated, usually during IVF but some people can get it with clomid too. It's most common in women with polycystic ovaries and younger women but not necessarily.
It is triggered by the hCG hormone, which is the hormone produced in pregnancy and what your pee sticks are looking for when you test. hCG is also what they use in IVF to "trigger" ovulation. The injections you do in the run up to egg recovery are to suppress and stimulate your hormones at the same time, making many follicles grow on your ovaries, but not be released. The trigger injection you do will cause the eggs to mature and be released, but you will have egg collection before they are, around 35 hours after your trigger injection. This trigger is (often) hCG and can cause your ovaries to become hyper-stimulated.
During your IVF cycle you will be monitored by internal scans and blood tests to see how your follicles are growing, and if there looks like there are many they will keep an eye on you as you will be at risk of OHSS. The problem is that once you have had your embryo transferred, there is every chance you will get pregnant, thus start producing hCG, which will make your OHSS worse.
The symptoms are intense pain in the abdomen, bloating, vomiting, diarrhoea, and feeling generally awful. You could end up retaining water in your abdomen, causing you to become very dehydrated, and your blood to be thick and clumpy and thus at risk of clotting. The water could collect in your abdomen and as high as your chest causing difficulty to breathe.
BUT don't worry, IVF clinics are very much on top of this. Even before I was showing any symptoms I was told they would put me on blood thinners as soon as I had my EC. I was told to drink lots and lots, and keep an eye on how much I was peeing. I felt pretty rotten for the first 3 days, but the blood thinners seem to be doing the job and I feel great now.
As I said, it's serious, but not very common, and the NHS especially are very on the ball with it so will look after you if it looks like you might be unwell.
I hope that's a good explanation but please correct me if I got anything wrong!