At the beginning of my 20 flushes I had exactly the same what you describe .
However, I seriously doubt that the "stuck thing" is in the colon and would guess it is in the liver. At least this was the case in my case.
And according to me it is very easy to undestand why:
At the beginning of flushing the liver is full of stones. Being pushed out by means of Liver-Flush induces the other stones to move forward and clog together - thus obstructing the liver ducts. Between my flush 6 and 7 this obstruction occured withing a few minutes. From feeling extremely well I plunged into a bad nausea and colon cramps.
The anser is: Flush as quickly as possible. As a matter of fact although not according to the textbook I did several LFs in 1 week intervals because of this feeling that something was stuck.
The "light" alternative would be: Drink one liter of water first thing in the morning and as much as possible of a 50:50 lemon juice olive oil mix thereafter. This should give you relief.