Sounds over carbonated. You added priming sugar which would have resulted in CO2 production. You didn't say how long after the 2 weeks of natural carbonating you stored it cold but once you did that the beer started to absorb the CO2 in the headspace.
After that, you then mechanically forced carbed it the way you did (force carbonating is any use of CO2, not just shaking), which resulted in more CO2 being absorbed by the beer.
Unhook the beer from the CO2. Purge the keg a couple to a few times per day for the next 2 days and retry the beer. As you purge the keg the beer will release CO2 to bring the headspace into equilibrium. If it's still too carbonated keep purging.
A couple other thoughts...
Serving with too short of a line will result in beer turbulence resulting in a release of CO2. Also, as the beer warms in the line it releases CO2 resulting in foam.