New ceiling light not working
It's possible you have a short.  From your question I don't know if you disconnected D by removing the connection at C or D.  You could try this:  

I'll assume since you did all this you're basically OK working with electrical.  Get yourself a non contact voltage detector and use that through this whole process to confirm things are dead before you touch them.


With the breaker off, and the switch off, look at the wiring in the box at D.  Look for any place the bare conductor of the hot wire could touch the bare ground wire, the neutral, or metal inside the box.  Look for nicks in the wire, places where stripped wire extends past the wire nut, etc.  
If that doesn't reveal anything, with the breaker and switch still off, remove the last light fixture (D) and cap the hot and neutral wires with wire nuts.  Turn on the breaker, turn on the switch, see if it trips.  If so, there is probably a short between the hot and neutral wires or hot and ground wires in the cable between C and D.  
If it trips with the D light disconnected and the wires capped, turn the breaker and switch back off, remove the C fixture, undo the wiring and cap the wires that feed D, and reconnect C as if it was the last light in the line.  Turn the breaker back on, turn the switch back on, and confirm that the breaker doesn't trip.  


If it does, then something's not consistent.  I'll assume it doesn't.  


Turn off the breaker and switch again, and test for continuity between the hot and neutral wires from C to D, and between the hot and ground from C to D.  If you see continuity with the wires disconnected and capped at both ends, you've found a short.  The wire from C to D will have to be replaced.  


If this doesn't lead you to a fix, you might want to call in for reinforcements, and leave that breaker off until they arrive.