If you've had rads and chemo you could have some lymphedema (doesn't only happen with neck dissections) there is always damage to the area being radiated, and fibrosis. There is also a lot of healing going on. Your body's reaction to damage is to fill the area with fluid - blood and healing elements that can also cause swelling and the damage from the radiation can make it hard for that stuff to circulate. And fibrosis can cause hardened and stiffened areas. Gentle stretching at this point isn't usually a bad thing. The usual signs of infection are hot spots (areas that are hot red and sore) and of course if it's a raw area there would be pus etc...

You've had a scan and are clear so maybe a bit of gentle massage?

best of luck... try not to worry.