lost of mass conversation
 

I am doing computation under Fluent 6.3 to model blood flow in arteries.
recently, as I wasn't satisfied with my results, I decided to check my udf with a simple cylinder.
Among different tests, I set a constant pressure outlet condition directly under fluent, and by changing one udf code, to ensure that the results will be the same.
the test was done for steady case.
I have been really surprised by the results i obtained: for a pressure condition of 0 at the outlet, everything is correct, but if I set a non 0 value ( I tried with 2000, 200 and 1) the mass conservation is not respected anymore (the difference between inlet and outlet volume flow rate is in the order of 1000% at least!). I have done the same test with my patient specific mesh, containing 19 outlets and 1 inlet, and there the mass conservation is respected.
I also tested with another cylinder, with a finer mesh, and then again, the mass conservation was lost (for a pressure of 2000 at the outlet, the outlet flow rate is 2x10^3 times the inlet flow rate).
I am wondering if this can be due to the fact that, for any reason, fluent consider the fluid as compressible. I read through the manual, but couldn't find a precise way to set or unset the compressibility of the fluid, apart from selecting ideal gas in density, which i haven't done.
If you have any idea about why I have this situation and how to tackle it, that would be very appreciated.
thank you very much.

edit:
I don't understand what went through my mind, the title should be read "lost of mass conservation" instead of "lost of mass conversation" of course.