@destiEos, hang in there. The good news is: you are on the 5th centile, not below third yet, where things tip into the severe end of the scale. You are also late-onset, which means that baby has had a good chance to do some growing already. Both these things are really positive.
The things they discussed with you are the standard elements they discuss with patients with babies with short femur length. I know this, because I am in the same boat. Firstly, it can be just coincidence (especially if you are not very tall yourselves); it can be IUGR (growth restriction - which means they will likely check the blood going through the umbilical artery and uterine arteries, and likely do a screening to see if you picked up any infections); it can be dwarfism (achondroplasia) (but then usually there are other signs, such as bent bones, etc.); and it can be a soft marker for things like Down's, where again, there are usually other markers as well (like a high score in the combination test).
At the moment, it sounds like they are giving you all those four 'could be's', and are simultaneously trying to tell you that the third and fourth are less likely in your case, due to the absence of other elements pointing at them. The first two explanations explain the vast, vast majority of cases. They will try to figure out in the next few weeks what is causing the short femurs, usually through a process of elimination. So, some tests will come your way, including frightening sounding ones. All of this is routine, and part of that elimination process, in which they also need to exclude the more rare causes.
I know this is very scary, but considering how far along you are, you are doing ok. Being at FMU, means you are in the best hands. If it helps, a FMU consultant told me, that making it to 32 weeks is really great, as the main foetal development has then already occurred. For us, reaching that milestone is our main objective. So you have already accomplished the key bit. You and baby can do this! 