two bedrooms in my house have developed the same problem. the return in each sucks are normally, but additionally the vents don't blow heat, instead suck air in a little. not as much as the return though. all my "output" vents at the furnace are connected. all other vents connected to furnace are ok.
Something had to have changed in the system like someone changed some damper settings. Try closing down on some dampers or registers to increase static pressure on the duct system. This should help. Make sure you mark the oridginal damper settings so you can always put back as it oridginally was.