Sudden Aggression/Mounting, Increased Jealousy

Question
Hi, I have a three year old lab mix named Jack that I got from the local shelter about two years ago.  He is neutered, and has been for as long as I've had him.  He has been a very mild mannered dog, easily trained, very friendly, with little bumps in the road on occasion.  I move frequently, and have always been concerned with the moves stressing him out but he's never shown any signs of stress or discomfort.  However, with my latest move (a couple weeks ago), his personality has dramatically changed.  At the dog park (I always find the local dog park within a few days of moving, to keep him social and active) Jack has suddenly started attacking several dogs.  He's always been noisy, with play-growls, but these are not his normal play-growls.  Even with his leash on, he will lunge and pull while snarling and snapping at the dogs.  I've started walking him at the heel around the park for a few minutes before letting him loose, which seems to calm him down for about ten minutes before he starts attacking again.  When pulling him away from dogs, I've had a few close calls where, if I had not moved quickly, Jack would not have hesitated to bite my hand/arm.  This is NOT the Jack I know.  Along with this, Jack frequently tries to mount every dog at the park (new behavior) and I walked in on him mounting my poor kitty the other day.  There is also increased jealousy should I show too much attention to my cat or my boyfriend.  And if I even lay a hand on our other dog (who's remained un-changed through this move), Jack starts whining incessantly until I stop petting the other dog.  At the dog park, the whining is replaced with growling if I show attention to another dog. Health wise, Jack is up to date on vaccines/fleas/de-worming, but he has been losing a little weight lately.  Also, I am about four months pregnant now, but I have not changed anything about my lifestyle in regards to him.  He gets as much attention as he used to.  I am concerned about the aggression with a baby, reasonably.  Any ideas on what's up with my Jackimus?

Answer
Frequent moves, combined with your pregnancy (dogs can smell the pheromones involved in pregnancy) and your apparent (not intended) "promotion" of Jack by stopping your attention to the cat, the boyfriend involved (understandably) and your obviously unintentional anxiety in situations where Jack is demonstrating control issues, all contribute to a serious problem.  His losing weight may be a symptom of illness or a symptom of stress: YOU NEED TO TAKE THIS DOG TO THE VETERINARIAN.  Regardless of what the cause, whether physical or emotional, this dog requires the in person evaluation of a certified applied animal behaviorist.  BECAUSE OF THE SOON TO BE BORN BABY, you cannot take any chances.  This dog needs a hands on evaluation and you need education regarding how to put Jack in his place psychologically (more complicated with another dog in the household) and how to prepare him for the baby.  DO NOT HESITATE to seek out professional help (IN PERSON, not on the internet.)  Contact the veterinary college in your geographical area and ask for referral.  Take the dog to the veterinarian, there may be a developing physical problem (joint related, neurological, etc.)  BUT SEEK PROFESSIONAL HELP with his behavior.