Wet suit fit

Hello all... I am brand new to the sport and had a question about a wet suit that I recently purchased. The suit in question is a Orca S3 suit. The size is medium large based on the fact that i am 6 ft 177 lbs. 

Now for the issue... I took the suit to the pool today to try it out in the water before I used it in an upcoming race. It fit tight outside of the water and the range of movement in the arms was acceptable. Once in the water I began to swim and everything felt great with the exception of a little drag in the back of the neck area. It seemed as if it was gathering some water (wasn't sure if that was normal) I continued swimming and was on pace to knock a full minute of of my 500 meter time and then my left arm seemed to feel strained. I continued swimming with the pain for the first 500 meters then stopped. I inspected the back of the suit and everything was zipped up (i think) correctly and began swimming again. about 200 meters in I started feeling the flooding and the pain in the shoulder area again. I stopped took the suit off and continued swimming and had no pain or strained feeling for the rest of my 2000m swim. It seems as if the pain in the arm was caused by something that is wrong with the wet suit but I have no idea if it is a bad size or if I had put it on wrong and left the neck flap in the back unsecured. Does anyone here have any idea as to what could be causing the issue? Thanks in advance for your help.