When you say doc do you mean your primary care physician or your surgeon? If the first, then see your surgeon. This is not at all uncommon and could be one or both of two things which are a) inflammation of the ligaments around the joint or b) adhesions between the ligaments.
a) needs rest, icing and pain killers and anti-inflammatories will help as well - ease up a little on the physiotherapy but don't quit it completely. Talk to your PT about it as well.
b) adhesions - all the structures in the body, muscles, tendons, ligaments, even gut and lungs, need to be able to glide over one another smoothly to let our bodies work. To do this, the body produces a special viscous fluid that acts like the oil in your car engine and lubricates everything with great efficiency.
Now when the surgeon opens up any part of our body, the internal stuff gets exposed to the air which is, by comparison to the body, cool and dry. The natural reaction is that the fluids evaporate and the tissues cool. This can make the organs and structures dry so when the wound is closed, part of the healing process is for the body to go into overdrive, producing extra fluid to replace what is lost. This is one reason why you need IV fluids during and after an operation.
Now as a general rule, 95% of people manage to make up this fluid loss and normality is restored quite quickly. But in the other 5%, for a variety of reasons some known, a lot unknown, their bodies are deficient in making up this loss and lacking the necessary lubrication, the ligaments and muscles lose their 'glide-ability' and beging to stick to one another. This is what we call adhesions. (Adhesions are often mistakenly referred to as scar tissue even by doctors.) If this happens and the function of the joint is affected, the answer is to work the joint with some force, thereby freeing the structures from their locked-in state. Much as you would if you got a sticky or rusty lock and put some oil in it, you'd work it back and forth to free it up. That's what a manipulation is.