Well, if it was a clot (and obviously I hope it isn't!) you shouldn't be waiting that long to be seen. Can't you call NHS direct? Or the prenatal day assessment centre at your hospital?
Signs it might be a clot (you may have some or none or all of these): it hurts, it hurts when you pull your toes up towards your knee, you have a rash or redness or the leg feels hotter, you haven't injured yourself but you have swelling. My DVT didn't hurt, btw.
In terms of the breathlessness, some people have a cough, pain when breathing, cough up blood. I didn't have any of those. My main symptom was extreme breathlessness, worse when lying down, breathlessness at rest - not just on exertion.
Clots are quite rare but if you do have the symptoms you ought to be seen. Most people get checked out and they find the real cause is something like accidental injury whilst sleeping, phlebitis, lack of iron or a number of other issues... So there is no need to worry but personally, I would run your symptoms by a medical professional just to be sure and safe. If you are in the UK you will normally have had a DVT risk assessment at the beginning of your pregnancy and the details are in your notes. So they do take the topic very seriously.