If I'm remembering right, was Alfie was a bit of a grouch with the rescue and when you first brought him home?
Perhaps his pain issues are longstanding, and may even have led to the problems in his former home that led to his rehoming? It might be worth mentioning this to the vet, so that he can take it into consideration as he tries to get to the bottom of the problem.
It is not all that unusual for a dog to go through a "honeymoon" period, during which the adjustment to their new home over-rides the dogs habitual/normal behaviour - if Alfie had learnt how to behave in a grumpy/aggressive manner in his old home (due to pain or discomfort) then he may have reverted to this behaviour now he has settled in; the pain relief may well have revealed the behaviour of a pain-free Alfie, which he may not have experienced for some time
If your vet is supportive, you could ask for a referral to a chiropractor while you are waiting for the blood test results? Our vet found nothing *wrong* with Molo, but Vav (his McTimony Choripractor) discovered that his pelvis was misaligned and had been uncomfortable enough to put him off being his usual active, agile self
I would also recommend Bach Flower remedies, which can be bought in most Health Food stores and are safe to use in combination with other conventional medication, in particular, Star of Bethlehem which is excellent for all rescue dogs to help them overcome the past and move on. It is excellent used alongside re-training or rehabilitation of a rescue dog, and may well help Alfie if his pain is long-standing as it will have left him worried and anxious as well as in pain
I hope you get to the bottom of Alfies behaviour very soon