Based on my previous experience..the reverse flow occurs if wrong did not define the outlet correctly. Changing the outlet BC might fix this problem. You can also increase the dimension of the problem (longer) to reduce the effect of reverse flow. (Just my 2 cents)

Two thing may be the cause, in my opinion: the inlet was made negative (aspiration outward) or a mistake is made in the choice in the outlet B.C. should be reviewed the manual of the Fluent code, there are plenty of example for these cases.

Reverse flow occurs when settings on Outlet are wrong. You should try what Aizat Abas told you, increase your flow field from the sourfaces of interest. Also check your residuals while performing your simulation, tending to zero is a good signal, otherwise you should verify your initial settings.   

Regards, Enrique.