sounds like you have an air pocket. check the level cold again, hopefully you will be able to top it off. squeeze the upper hose a few times while fillling. if something weird didn't happen during the repair, its probably just air trapped. top it off while its running, keep adding little bits untill no more bubbles or burps, and the fluid is to the very top and it will spill out when you put the cap on.

 

Sound advice. When it cools down you could also pull your top heater hose off, by the valve cover, and try to fill the system until you get coolant coming from there. This usually gets most of the air out of the system.

If your worried about spilling some coolant the parts stores usually carry a rad filler funnel that you leave on in place of the cap to help purge out the trapped air. With this funnel you run the engine for about 15-20 minutes then let it cool down, top it off if needed and put the rad cap back on.

 

If you find no air pocket could be a bad thermostat

just my 2 cents