Unfortunately, cancer can recurr at anytime, although not common in several weeks, or can be reminiscent, but the symptoms may just as well be treatment related, and as the doctor said, the morning swelling can be from the lymph fluid pooling. I sleep on an incline, which I find helpful. I'm almost 4 years out from radiation, 3 years from my first neck dissection, and still have swelling, more so on the most radiated right side, and under chin. Everyone is different, had different Tx, and it's not easily understood why some have lympdemia while others don't, but it's not uncommon. Some do PT, have manual lymphatic drainage massage, which may help prevent fibrosis, but check with your doctors first. I had so many recurrences I wasn't letting anyone play around with my neck, except my ENT.

Good luck with the MRI. The majority of post treatment scans at three months are clear of cancer.