I Recently changed heatercore and everything was working fine.
A week later the heater blower starts working intermittently, but now I am getting nothing but cold air.

Any Ideas

The blower not working is a separate issue. Your problem with the cold air is likely air in the system. Make sure the coolant is topped up and let the car idle with the reservoir cap off and the heat control on full. You need to work out the air pocket. If that doesn't work, loosen coolant hoses and squeeze slightly to try and work the air out.