I had surgery in March and I think it felt easier after about a month. You are doing the exercises so that will help and perhaps the swimming would be good exercise too. Don't over do it though.
I'm about 5 months down the line and still get a bit of discomfort under the arm, especially in evenings. I also find if I have been lifting that it aches a bit.
I had surgery 2 weeks ago, lymph nodes clear thankfully, but the tightness of the skin around my armpit is hugely irritating! In the grand scheme of things it sounds trivial, but I just wondered how long it is before the tightness starts to ease. I am doing the exercises 3 times a day as recommended. I can handle the discomfort to a point and intend to start swimming again soon. Just would be nice to have an optimistic time scale to aim for!